El corralon is a Spanish phrase meaning the big corral. In the Rio Grande Valley, El Corralon is a detention center run by the Justice Department, a reminder that not everyone who gets to America is legally entitled to America's freedoms.

Illegal immigrants are held here, dressed in bright orange jumpsuits, restrained by electronic gates and barbed-wire fences, monitored by closed-circuit security cameras. It is a desolate place at the end of a farm-to-market highway, surrounded by an alligator swamp, about 20 miles inside the Texas border. The land of opportunity they sought, the dream that carried them across Mexico and the Rio Grande, can be seen only if they look east from the third-story windows of their bunkhouse and, in the far distance, glimpse the high-rise luxury condominiums of South Padre Island.

Of the 12 detention facilities for illegal immigrants in the United States, known as processing centers, this is the largest. It holds between 400 and 685 men and women, most of them Salvadoran and Nicaraguan refugees seeking political asylum, which a minuscule percentage of them receive.

They are not convicts. The laws they broke are the administrative rules of the U.S. Immigration and Naturalization Service. But they are held here, for periods ranging from two weeks to six months or more, for interrelated reasons: They have rejected U.S. offers to be flown home; they have requested asylum or are trying to delay deportation in other ways; and they cannot afford bail, usually $3,000.

El Corralon, a symbol of confinement, feeds a nearby religious sanctuary, Casa Oscar Romero, a symbol of freedom.

When the Border Patrol catches more Central Americans than El Corralon can accommodate, or when the processing center's budget is strapped, the doors of the big corral open and some people are let out. But they are ordered to stay in the area, below the highway checkpoints at Kingsville and Falfurrias, and many of them, inevitably, end up at the only other place they know, Casa Romero.

In August, when a record number of Central Americans were caught in the Rio Grande Valley, there were 685 detainees at El Corralon and 300 refugees at Casa Romero. Then authorities began releasing large numbers of immigrants on their own recognizance and, for a time, there were more at the four-room sanctuary than at the center.

"The people who manage to get out of El Corralon are getting trapped down here, where they have no job prospects, no family, no friends," said Jeff Larsen, a lawyer for Proyecto Libertad, which represents Salvadorans and Guatemalans seeking political asylum. "The INS is using the whole Valley as a detention center. They have pushed out the fences of the corral."

In the lexicon of the Border Patrol, the immigrants held at El Corralon are OTMs -- Other Than Mexican. Their world is divided not into East and West or repressed and free or developed and underdeveloped, but into Mexican -- 85 percent of the illegal immigrants caught each year -- and Other Than Mexican. Mexicans caught in the Valley are taken back to the nearest border crossing and released. Other Than Mexicans are processed, detained and, if correct procedure is followed, given an opportunity to request asylum. Miroslav Medvid, the unlucky Ukrainian ship-jumper whose plea for asylum was misunderstood and rejected by the Border Patrol in New Orleans, was recorded as Other Than Mexican during his brief sojourn on U.S. soil. His treatment more resembled that given a Mexican.

There are no Soviets at El Corralon now, but there are a few Yugoslavs and Poles who were caught in the south Texas brush country after alien smugglers, known as coyotes, led them up from Mexico City and across the Rio Grande at a cost of $3,000 per person. Border Patrol agents say the eastern Europeans are not very hard to spot: a band of pale-skinned people wearing odd clothing and clumped together near the highway or moving along a field, unable to converse in English.

Most of the center's detainees are from El Salvador and Nicaragua, where deprivation and political violence have become parts of life. During the last week in October, the roster included 132 Salvadorans, 87 Nicaraguans, 40 Hondurans, 31 Colombians, 18 Guatemalans, 12 Uruguayans, 10 Nigerians and one to five each from Belize, Ecuador, Costa Rica, Lebanon, Peru, Thailand, India, Poland, Yugoslavia and India.

The place is a miniature United Nations of American unwanteds.

On a recent afternoon, Marcelo Chavez, a Honduran fruit picker, stood in line for a medical examination. Chavez had abandoned his legal efforts to stay and was waiting to be deported. He had waded alone across the turbulent Rio Grande on the afternoon of July 4. He was caught later at one of the highway checkpoints further up the Valley.

It is relatively easy to sneak across the river without getting caught, for in this one sector alone, from Rio Grande City to Brownsville, there are 270 miles of riverfront and 236 Border Patrol officers, less than one per mile if they were stationed along the river at the same time, which is never the case. But moving out of south Texas is a more difficult maneuver. Only two roads, Highways 77 and 281, lead north out of the brush country, and both are guarded by the Border Patrol 24 hours a day. Chavez was caught on his way to New York.

In the laundry room, folding sheets, was Evart Garcia, a gold-toothed Nicaraguan who had been at the center for two months while the INS processed his request for political asylum. Garcia said he left Nicaragua because "it has a communist government." The Reagan administration shares that assessment of the ruling Sandinistas, but the statistics do not favor Garcia's asylum request.

Only 12 percent of the Nicaraguans seeking asylum here have received it, and the percentage is far lower in the Rio Grande Valley. Federal officials here do not expect that to change. "We treat them just like any other Central American," said J.L. Hicks, deputy director of the Border Patrol's McAllen sector. "I'm sure we all have compassion for them, but we have to set limits. We have to draw the line somewhere to preserve the country we have now."

Antonio Mendez, a mechanic from El Salvador, was out in the midday sun, bouncing a soccer ball off his knee. The Border Patrol caught him one night in mid-September in a grapefruit orchard on the Brownsville side of the Rio Grande. People were disappearing in his home province, Mendez said, and he decided he had to leave. His sister was in Houston. The closest he will get to her, in all likelihood, is the Houston Intercontinental Airport, where an INS bus will take him to be placed on a commercial flight to San Salvador.

To stay in the United States, an immigrant must provide documentation that he would be persecuted if he returned home. Lawyers at Proyecto Libertad have never won an asylum case in the Rio Grande Valley. "The best we can hope for is a change of venue," said Larsen, who spends much of his time at El Corralon trying to free refugees on bond and appearing before two INS judges. Larsen said that, with luck, "we can get them out of the Valley, to a city where they have relatives. If we can help them get to Washington, that is a victory."

"It's simply that those denied asylum don't have good cases," said INS spokesman Duke Austin. "They don't have proof of a well-founded fear of persecution if they were returned home. They may have a good case for random violence, they may have a case of economic troubles, but that doesn't get you political ayslum in the United States."

In the women's dormitory, Rosa Estel-Mana, a 28-year-old Colombian, was walking out the door in civilian clothes. She was being deported after spending two months at the center. Soon she would be reunited with her eight-year-old daughter, whom she had left with a grandmother at home. Her immigration route involved a flight to Mexico City and a bus trip to Matamoros. She had crossed the Rio Grande Aug. 7 and was caught a few hours later at Valley International Airport trying to board a flight to New York.

Esmeralda Herrera, 21, was leaving El Corralon this day, too, but had been bonded out by her husband, Manuel, whom she would join in Miami. Manuel Herrera left Nicaragua last April and rode buses to the border. He was caught crossing the Rio Grande and detained at El Corralon, but his bond was posted by an aunt in Miami.

The family network was working for the Herreras. And in Miami, with its strong, anticommunist Cuban community and a growing Nicaraguan one as well, the young couple stands a better chance of winning asylum or somehow delaying deportation.

There is a remarkable grapevine among immigrants fleeing north through Mexico. By the time they approach the border cities of Matamoros or Reynosa, most have learned that the surest way to avoid detention at El Corralon, if they are caught crossing the Rio Grande, is to be accompanied by a child, because most illegal immigrants with children are released on their own recognizance. Many cross the river with children who are not their own. J.L. Hicks, deputy director of the regional Border Control office, calls them "Rent-a-Kids." On Oct. 25, he suspected that one such child was sitting in a processing room of the McAllen station.

At 6 a.m., Jose Raul Padilla had entered the Rio Grande near Reynosa. He was a Nicaraguan, a former officer in Gen. Anastasio Somoza's national guard who had spent the last five years with the contras in the mountains of Honduras, working against the Sandinistas. The water was up to his knees as Padilla, 44, waded toward Hidalgo, Tex., pushing an inner-tube that carried a three-year-old boy.

Padilla said that he once thought President Reagan would change things in Nicaragua but that he recently abandoned such hope. The contras were fighting among themselves, he said, and an overthrow seemed unlikely. "One gets tired, real tired," Padilla said. He identified the child on his knee as his nephew, Jose, and said he took him on the journey through Guatemala and Mexico. Hicks was suspicious.

"He knows we're not going to detain him with a child like that," Hicks said, watching from a distance as Padilla was being photographed. "It might not be his child, but how are we to decide that?"

The appearance of Nicaraguans such as Padilla along the Rio Grande border is a relatively recent phenomenon, according to Hicks. Ninfa Krueger, director of the Border Association for Refugees from Central America, agreed. "Up through July of 1984, we saw an average of 40 to 50 Nicaraguans a month," Krueger said. "Since then, there has been a very noticeable increase. Before, we would see young men evading the draft. Now we are seeing all types . . . former generals, whole families. Our conclusion is that as the violence increases, more people try to leave."

Krueger's organization deals with refugees who have been caught and those who have not. The Border Patrol, whose statistics reflect only immigrants who were caught or surrendered, reported 131 Nicaraguans were caught last January in the Rio Grande Valley; 239 in February, 434 in May and a record 587 in June.

Whether El Corralon is filling up because more Central Americans are flowing across the Rio Grande or because of the recent addition of new Border Patrol agents is the subject of some debate on the border. The 1985 INS budget allowed for hiring 512 more agents nationally, 230 of them in south Texas, the main entry point for Central Americans.

Opponents of congressional immigration bills argue that the record number of arrests this year reflects troop reinforcements. The numbers are manipulated, said one such opponent, Gilbert Cardenas, professor of economics at Pan American University in Edinburg, Tex., so that the public will be left with the impression that aliens are swarming over the border. "There are just more people to catch them," Cardenas said. "The same people are getting caught again and again by more agents."

Statistics at the Brownsville subsector, the busiest bureau in the busiest sector along the 2,200-mile border from California to the Gulf of Mexico, contradict him. There were 21 agents at Brownsville in 1980, and there are 21 today. The number of immigrants caught there has nearly tripled, and the number of Other Than Mexicans more than quadrupled.

The principal U.S. response has been to construct more detention facilities. The newest one, which will be even larger than El Corralon, is nearing completion in rural Louisiana, near the small town of Oakdale. When it opens in January, the Oakdale processing center will be able to hold 1,000 illegal immigrants at a time. The INS expects 35,000 of them to move through the facility each year.

Oakdale is about four hours by car from Houston and nearly as far from New Orleans. The town has five lawyers, and none of them are known to speak Spanish. The American Civil Liberties Union filed suit, charging that the location denies immigrants an adequate chance for counsel, but the suit was dismissed in federal court in Washington.

"Hundreds of the people at El Corralon are getting deported before we can get to them to help," said Proyecto Libertad's Larsen. "Imagine what it will be like in the middle of central Louisiana. El Corralon is 30 miles from anywhere. Oakdale is truly in the middle of nowhere. It is as though these people are criminals without rights. That is how America treats them."