A study led by a Texas economist, in a direct challenge to prevailing opinion among U.S. scholars, has concluded that illegal immigrants are taking thousands of jobs from Americans and costing billions in tax dollars.

The 29-page research monograph was presented by Rice University Prof. Donald L. Huddle to an Immigration and Naturalization Service conference last week in San Diego. It argues that undocumented Mexican workers are no longer mostly seasonal migrants taking jobs Americans don't want but are often skilled or semi-skilled craftsmen earning more than $5 an hour.

Huddle put the net cost to taxpayers in unemployment, education and welfare expenses at more than $7 billion a year.

The survey was co-authored by immigration policy researcher Arthur F. Corwin and former U.S. Border Patrol deputy chief Gordon J. MacDonald. It escalates an ongoing effort to control the intellectual underpinnings of the legislative effort to regulate illegal immigration. It says the "myth" that undocumented workers represent an economic windfall "was a factor in the formulation in 1982 of the Simpson-Mazzoli immigration bill."

Although Congress failed to resolve differences over the legislation last year, sponsors Sen. Alan K. Simpson (R-Wyo.) and Rep. Romano L. Mazzoli (D-Ky.) have indicated that they will reintroduce measures to legalize millions of illegal immigrants and penalize employers who hire undocumented workers in the future.

The Huddle study suggests that legislators reconsider legalization proposals in light of what it says an underground immigrant economy has done to job opportunities for U.S. citizens in the southwest.

During past labor shortages, such as in wartime, "American employers, contractors and recruiters never hesitated to break through the barriers of structural unemployment -- and substructural isolation -- to hire minority workers," the report said. But with a stream of illegals from Latin America, unemployed black and Hispanic U.S. citizens are no longer "being courted, recruited and uprooted from inner city settlements and slums and brought into the mainstream job markets," according to the report.

The findings are based on interviews by Huddle's students with construction workers and on dozens of studies of immigrant behavior and government support in other parts of the southwest, particularly Los Angeles.

It criticizes what it calls the "windfall thesis," accepted by many American and Mexican scholars, that most illegal immigrants take jobs as migrant farm workers, urban laborers or servants that Americans do not want and that illegal immigrants pay more in sales taxes and unrecovered tax withholding than they collect in welfare and other government benefits. Huddle's team interviewed 200 undocumented workers in the Houston area and found that 53 percent were paid more than $5 an hour and 12 percent more than $6 an hour.

"Only 38 percent of the respondents were working as common laborers. The others were distributed among 14 trades, including foremen (4 percent), cement layers (15 percent), carpenters (14 percent), ironmen (8 percent), plumbers and roofers (each 2 percent)," the report said. In a study of highway and commercial construction workers, "we found that illegals commonly were paid between $8 and $9 per hour as insulators, concrete finishers and painters."

Based on the surveys, the Huddle group estimated that for every 100 undocumented workers "at least 65 U.S. workers are displaced or remain unemployed."

They also cited several studies indicating that illegal immigrants cost taxpayers money.

"For instance, the state of California in 1980 spent $3,254 on the average Mexican immigrant household, including legal and illegal immigrants, but in return received only $1,515 in tax revenues."

Other U.S. experts on immigration, asked for comment on the Huddle report, said they agreed that undocumented workers may be staying longer and earning more than in the past. But they said they saw little in the report to back up the thesis that American workers are being crowded out as a result.

University of Maryland economist Julian Simon, a prime target of the Huddle report, labeled "simply eyewash" the report's reference to "the furtive, sleazy underground economy of poverty-stricken illegal aliens." Illegal immigrants, he said, see their lives here as a significant improvement over their lives in Mexico, and they add jobs and dollars to the economy.

In an interview, Huddle challenged this view, saying that the construction projects he studied in Houston would have gone forward with or without immigrants.

Leo Chavez, research associate at the Center for U.S.-Mexican Studies at the University of California at San Diego, said he saw no data in the Huddle report supporting the thesis that 65 U.S. workers are displaced or left jobless by every 100 undocumented workers. He said the Los Angeles study showing Mexican immigrants taking more in services than they paid in taxes includes neither unmarried immigrants who demand little from the government nor federal taxes the immigrants may have paid.