A drive to organize illegal Mexican aliens working in Arizona's citrus fields has led to the formation of a new migrant labor union, the Arizona Farmworkers.

The union, whose incorporaton was announced yesterday in a news conference near Phoenix, is believed to be the first ever organized with the specific aim of helping Mexian nationals, so-called "undocumented workers" employed in the United States.

"Since 1977, undocumented workers in Arizona, especially in the citrus industry, having been openly striking, going to court, and protesting to fight for their rights," said Don Devereux, a board member of the Maricopa County Organizing Project, which played a key role in organizing the new union.

"For years, everybody assumed that undocumented workers could not be organized because they were too vulnerable to deportation. This is a major breakthrough because it shows those people were wrong," Devereux said.

At least half of the new union's 400 dues-paying members are illegal Mexican aliens, said Devereux. The others are Mexican nationals working legally in the United States and native American migrant laborers, he said.

Arizona state officials contacted yesterday expressed surprise that a union has been formed in their state with a large illegal immigrant constituency. But they said the presence of illegal aliens in their state, especially in the $160 million-a-year citrus industry, is a well-known fact.

"My latest estimates are that about 90 percent of the 2,000 or so citrus workers in Arizona are illegal Mexican aliens," said William Gibney, general counsel to the state Agricultural Employment Relations Board.

"If the citrus growers want to get their crops picked, they have to go elsewhere -- usually to the illegal aliens or legal foreign workers -- because many of our local people don't want to do that kind of work," Gibney said.

U.S. Immigration and Naturalization Service officials in Arizona could not be reached yesterday for a comment on the new union. However, in recent interviews with the state's news media, INS officers were critical of collective bargaining gains made on behalf of illegal Mexican aliens.

"It's certainly not going to make my job any easier," Herbert Walsh, chief Boarder Patrol agent for the Phoenix area, said in an interview with the Arizona Daily Star.

"I'm sure we're going to get a lot of static from the organization when we arrest the members," Walsh said, referring to the Maricopa County Organizing Project, which last January won a major collective bargaining agreement for illegal Mexican workers.

Devereux, who yesteray acted as spokesman for the project, said he hopes the union will not spark a new round of deportations of Mexican illegals.