When the Census Bureau undertook a test run of the 1980 census in Austin, Tex., three years ago, officials were determined to pay special attention to Hispanic neighborhoods - because accurate counting of minority populations is a priority concern for planners of next year's national head count.

Austin's Hispanic neighborhoods got special attention, all right, but not the kind the Census Bureau likes to advertise. Within days after the census takers made their rounds, Immigration and Naturalization Service agents swept through the same neighborhoods in raids that caught an unknown number of illegal aliens.

"It was just one of those unfortunate things," said Henry Smith, the Census Bureau's chief information officer. "There was no collaboration between our people and the INS; they just did it at the same time our test was underway."

The INS also says that the timing of its Austin sweep was coincidental. But Austin's Hispanic population seems to find such official assurances hard to believe.

"Our people," said Ben Reyes, a Mexican-American member of the Texas legislature, "are, from the beginning, afraid of the government and, from the beginning, afraid of the census. . . . That [Austin incident], certainly expands the fear that people have got here."

The Austin case represents an extreme example of a pervasive problem facing the Census Brueau as it gears up for the nation's 20th decennial census next spring: with distrust of government widespread among the populace, people may simply be afraid to answer the personal questions that the census will ask them.

"We've had these traveling hearings around the country on the census," said Michael J. Ferrell, staff director of the House census subcommittee, "and the one message that comes through every place is that people just don't trust the federal government. They don't want to tell the bureau about divorces or part-time jobs or anything, really, because they're afraid it won't be kept confidential."

Although census officials understand that attitude, they think it is unfair to their agency, which is armed with civil and criminal statutes prohibiting the release to anyone - government official or private citizen - of personal information from the national head count.

"They have an absolutely perfect record, as far as we can tell," Ferrell said of the bureau. "We've never found any case where somebody can prove that census information on an individual was leaked to anybody."

On the other hand, critics such as the American Civil Liberties Union point out that there is an inevitable tension between the bureau's dedication to personal privacy and its goal of making information available in a form that is useful to governmental and private data users.

This is illustrated by a famous case that came up early in 1942, just after the attack on Pearl Harbor. The War Department asked the Census Bureau for the names and addresses of all Japanese-Americans counted in the 1940 census.

This information was sought to enable the military to round them up for internment in molding camps.

Despite the intense anti-Japanese sentiment that the Pearl Harbor attack had aroused, the Census Bureau refused to comply, arguing that it could not provide information on individuals. But the bureau did refer the military to tables it had published showing the racial makeup of every neighborhood in the country, making it possible for the War Department to find most of the Japanese-Americans without seeing individual census replies.

The ACLU is critical, too, of the bureau's practice of keeping each individual questionaire, including respondent's names, on file forever. The mere existence of such records, says the ACLU, creates a possibility for abuse. And the census forms do not tell respondents that the answers will be kept in a permanent record by name.

The bureau says the records are kept to provide age and genealogical data for people who do not have birth certificates.

Whether or not there is just cause for public distrust of the census, the bureau recognizes that it exists, and that it threatens the accuracy of the 1980 count, when the government will try to tally every resident of the United States and its territories.

"To the extent people are suspicious, it's going to be harder for us to find them," says the bureau's Smith.