WASHINGTON -- Union leaders representing Environmental Protection Agency scientists and other specialists assert that agency managers and pesticide-industry officials are exerting "political pressure" to allow continued use of a family of pesticides that might be harmful to children, infants and fetuses.

In a letter to Stephen Johnson, EPA's administrator, the union leaders said scientists are being pushed to skip steps in their testing, and alleged that the "integrity of the science upon which agency decisions are based has been compromised."

The protest from unions representing some 9,000 EPA scientists and other employees about a pending agency determination is unprecedented and a professional rebuke to Mr. Johnson, himself a scientist and former assistant administrator in charge of the agency's program to test the harmful effects of pesticides.

EPA spokeswoman Jennifer Wood said the agency "has been reviewing all pesticides in question and applying new, stricter standards as required under the Food Quality Protection Act, with a specific focus on their effects on children's health." The agency had no specific response to the union leaders' assertions. Spokesmen for groups representing the pesticide industry didn't immediately return phone calls.

The letter said the agency faces an August deadline to re-evaluate a family of 20 organophosphate and carbamate pesticides,many of them stemming from World War II research on nerve gas. They include malathion, commonly used to kill mosquitoes, and a variety of other chemicals that are used in agriculture, gardens, on golf courses and on flea collars and pest strips.

Under the Food Quality Protection Act passed in 1996, the EPA is required to review acceptable limits for residues of existing pesticides within 10 years. Organophosphates, which attack the nervous system, were among the group given the highest priority for testing.

Jeff Ruch, executive director of Public Employees for Environmental Responsibility, whose members include state and federal employees, said "the fact that this letter had to be sent at all is an utter disgrace."

Jennifer Sass, a toxicologist for the Natural Resources Defense Council, another environmental group, said there is "a lot of uncertainty" in scientific data about the pesticides and "newer, cleaner alternatives" are available. "This is old style chemistry and these [chemicals] should have been buried years ago."

After World War II, scientists discovered that insects were more sensitive to nerve gases than humans and it was felt that humans wouldn't be harmed by relatively low applications of the chemicals. According to a recent report by the EPA's Office of Inspector General, however, later studies showed that some pesticides can easily enter the brain of fetuses and young children and may destroy cells in the developing nervous system.

Although the federal law gave the EPA 10 years to settle the issue, the inspector general's report, issued in January, said the agency still lacked a standard evaluation procedure for testing the toxicity to developing nervous systems. The union leaders recommended that Mr. Johnson tighten restrictions on use of the pesticides until the questions are settled.

The letter was sent by leaders of nine local chapters of three unions: the American Federation of Government Employees, National Treasury Employees Union and Engineers and Scientists of California.