Carlos Solero couldn't believe his ears when his bosses started reprimanding him for speaking Spanish.

"Even if I was singing to myself or just mumbling, I would get a warning," the 31-year-old said of the "English only" policy his bosses instituted at a suburban Chicago manufacturing plant two years ago.

The company said the policy was needed to improve communication on its assembly line. Eventually, Solero was asked to sign a work agreement that included the policy--but he refused and was fired.

Now he and seven other Spanish-speakers--and the federal Equal Employment Opportunity Commission--have filed lawsuits against Watlow Batavia, a metal-casting and assembly plant in Batavia, Ill.

Legal experts say it is part of a growing movement against employers who try to force workers to speak English on the job.

"I think we're going to see more and more of these types of lawsuits, especially as the population of immigrants grows and more languages are spoken in the workplace," said Jose Behar, an EEOC attorney handling the agency's lawsuit against Watlow.

Complaints filed by the EEOC against companies with English-only policies have nearly tripled in the past three years, according to the agency. Ninety-one were filed nationwide last year, compared with 32 in 1996.

Companies may have English-only policies as long as they can prove they are a "business necessity"--say, in an air-traffic control tower.

Trouble is, say Behar and others, some companies are taking it too far and instituting policies simply because they want to know what their employees are talking about.

"When there's some type of health and safety at stake, those are situations where English-only should be required. But in the vast majority of jobs, there really is no justification," said Donya Fernandez, an attorney with the Language Rights Project in San Francisco.

This summer, the group helped work out an agreement at an Emeryville, Calif., casino where kitchen cooks were told they had to speak English, even though they all spoke Cantonese.

Officials at Watlow defend their English-only policy, which they say was temporary and part of a plan to improve a department in crisis.

"We have nothing against speaking any other language as long as the customer's needs are being met," said Diana Rader, the human resources manager for the company. "But the department was experiencing problems where we thought communication was only one of many issues."

The cases against Watlow Batavia are expected to be heard in U.S. District Court in Chicago early next year.