From behind the battered counter of her little general store, Lidia Suarez, 25, looks out on a ghost town.

Occasionally, a woman or a child passes through the sun-bleached square, but almost all of the men of the village have gone. They took clandestine paths to find work across the U.S. border hundreds of miles to the north. Nobody is ever sure when or if any of them will come back.

It was never expected, certainly, that the son of one of those long-departed men would someday return as the U.S. ambassador to Mexico.

Before Suarez noticed the visitor with his small entourage last weekend, she was saying, "I'm almost always alone here. Everyone is gone. There is nothing for them here." she laughed. "I would have gone, too, but on one took me."

Then, the ambassador was before her. He was friendly, speaking Spanish, his face showing Indian ancestry much like her own. But Lidia Suarez, suddenly finding herself the only welcoming committee that Susticacan had to offer, stood trembling and nearly speechless.

Julian Nava, the first Chicano to serve as U.S. ambassador to Mexico, stayed only for a few minutes in Susticacan. Nearly destroyed by the brigandage of the Mexican revolution's final days when his father left 60 years ago, its population has been decimated by emigration since then -- from about 12,000 in 1906 to about 1,200 today. As the town withered, Nava's roots here died with it.

The ambassador went on to Tepetongo, a larger, slightly more prosperous village where his mother was born and the townspeople were expecting him.

Nava was met with embaraces. An enthusiastic band echoed welcome through the dusty streets. Old men in sombreros turned out to watch as he passed by.

The scores of first, second and third cousins that still populate the town greeted Nava as a hero.

"Yes we're all very happy he has done so well," deadpanned one nonkinsman, "especially his relatives."

Yet through all the festivities, all the warmth and laughter and welcoming warmth and laughter and welcoming back, there was no mistaking the fact that whatever blood flows through his veins, Nava no longer belongs to these villages. He is an American. While his parents were from here, he is from Los Angeles.

The cultural chasm between those Mexicans who go to the United States and those who stay behind grows quickly and wide. Although Nava's interest an intelligence allow him to bridge the distance more easily than most, the distance more easily than most, his position makes him a symbol of all that is different between Mexican-Americans and Mexicans.

Nava sees the problem as a simple contrast between city dwellers and country people. He has few problems establishing a rapport with his Mexican friends and relatives who live in Mexico City, he suggests. "Where you find the gap is in Tepetongo. It's because we're urban. You find substantially the same thing with somebody from New York going to visit a cousin in Kansas he hasn't seen in 20 years."

But others see the contrast as more profound and a source of potentially serious difficulties as Mexicans and Mexican-Americans have begun in recent years trying to work toward common goals.

"The Chicanos do not understand the Mexicans and the Mexicans do not understand the Chicanos," said one Hispanic activist who has worked closely with both groups. "If they want to help each other they are going to have to realize their differences and quit assuming that because they share a common ancestry and language they share anything else."

It is not that most Mexican emigrants go north intending to stay or intending to lose contact with their people. Recent studies suggest that most eventually return to their villages. at least for awhile. But many, although they can go home again, cannot be at home again.

A government delegation from the state of Zacatecas, where Tepetongo and Susticacan are located, recently went to Los Angeles to promote the participation of the Zacatecans living there in development of their homeland.

One of the Zacatecan officials said he was stunned at how out of touch with the needs of their old villages the people north of the border had become. "the main thing they were interested in was better phone connections so they could call their relatives more easily," said the official. "And there was a demand for new baseball fields.

"These people in Los Angeles feel lonely and out of touch," said the offcial. "But when they get back they don't want to stay here. They feel proud to return with a new car and spend a lot of money at the fair, then they don't hve any money. They put another child in the woman, or another and another, then they leave again."

By some unofficial estimates, as many as 400,000 Zacatecans, nearly a third of the state's current population, now live in the United States.

For years, as the country became a virtual nation of emigrants, many Mexican officials tried to dismiss the problem as only of concern to the United States. But in areas like Zacatecas there is now growing belief that what was once thought a safety valve for the pressure of overpopulation and underdevelopment may in fact be a drain through which the area's social structure is disappearing.

The government recently has begun to provide better financial protection for farmers subject to the often vicious caprice of climate. Old silver mines are being resurrected and are expected to produce many new jobs along with their precious ore. There is serious talk of attracting industries.

But the officials are now up against a pattern of migration so strong it is providing difficult to break.

"Right now the people here have got more chances in the argicultural field than ever," said an adviser to the governor of Zacatecas. "But they keep going to the United States. It's like a tradition passed on from father to son. It's a matter of pride, even something to impress their fiancees."

Serious resentments, suspicions and prejudices have grown here against Chicanos. The Mexicans speak of them in disparaging terms.

Nava's post and his parents place him at the center of all this. When his appointment was announced early this year, the Mexicans made it clear that while they would accpet him as ambassador, they had no special reason to want to Chicano in the office, and would actually rather not.

Later some Mexicans came to think Nava would view the world and Mexico's relationships with it as they do. When he arrived and hewed close to the State Department line, some Mexicans considered him almost a traitor.

Such difficulties were widely predicted, but the Carter administration appointed Nava anyway because of intense pressure from Chicano organizations to name one of their own to a significant post.

The ambassador has had to strike a delicate balance between his heritage and his responsiblities. Some of his staffers privately voice concern that he says more than he has to in public, and perhaps more than he should. Despite the personal emotion that sometimes lies behind his remarks they are almost always founded in serious analysis.

On the migration issue, for instance, Nava told a reporter after visiting Tepetongo, "Sometimes it brings tears to your eyes when you see a beautiful young cousin and there are no men for her."

At a small press conferences in Zcatecas he noted that when his father and mother came to the United States, the border essentially was open for supply and demand to determine how many people should cross. He said that as long as the workers were registered and could be kept track of, he would favor such a situation today.

Some of his staff were against Nava, a politician and college administator in Los Angeles before he became ambassasor, believes from research and first-hand experience that some sort of extensive guest-worker program is the most fair and viable means of coping with immigration to the United States.

"There ar problems with it, sure," he said. "But I say, show me a better plan. You're not going to keep them from going."

Although Nava was born in the United States, he has maintained ties with relatives in Tepetongo since the early 1950s and he and his sisters helped raise money to build what is now its first high school.

Finally, it may not be so much for his heritage as for his interest that Nava is respected in these parts.

"Listen to the way they talk to the ambassasor," said a Mexican official as he watched Nava picnic with his relatives. "They're reaching for someone to help. Anyone."