For 20 years, Steve Strong has been raising peaches, plums and nectarines in California's San Joaquin Valley, relying on a pesticide called methyl parathion to control worms and scale insects. "It's a very useful tool," Strong said. "It gives us a hammer to halt an economic disaster."

So along with thousands of other growers, Strong will be paying close attention today when the Environmental Protection Agency is expected to announce several thousand new limits on the amount of pesticide residues allowed on food. Among the announcements, the EPA is expected to ban most uses of methyl parathion and a related pesticide on fruits frequently consumed by infants and children.

The restrictions stem from the Food Quality Protection Act, which mandated a broad overhaul of pesticide regulations to better assess and prevent risks to public health, particularly in children. The law directs the EPA, for example, to apply "an additional tenfold margin of safety" for infants and children except when there is "reliable data" that a less stringent standard would be safe.

Born in a rare burst of bipartisanship on Capitol Hill -- it passed both houses of Congress in a period of eight days in 1996 without a dissenting vote -- the law has been the subject of furious infighting ever since as the EPA has moved to implement it.

Methyl parathion and the roughly four dozen other "organophosphates" account for about half the pesticides used in the United States. Because they are so widely used -- on grains, vegetables and fruits, as well as on pets and to control mosquitoes and termites -- and because of longstanding concerns about their safety, the EPA has made a priority the review of organophosphates, including the methyl parathion used by Strong on his 2,000 acres of fruit in California.

The stakes for both industry and the general public are huge. Depending on how tough the EPA is in restricting their use, farmers and other users may have to switch to more expensive alternatives.

A year ago, alarmed by the direction the EPA was taking, the American Crop Protection Association warned that "sooner or later, virtually all pesticides and pesticide uses will be jeopardized."

Then three months ago, all seven environmental and farm worker groups serving on an EPA advisory panel on the reassessment process resigned en masse. The pesticide industry and agribusiness interests, charged one environmentalist, had "hijacked" the process.

The intense battle over implementation of the new law is a classic example of how the complex process of agency rulemaking is often far more important than congressional legislating, and of how difficult it is to translate a relatively simple goal -- in this case ensuring Americans their food supply is safe -- into practice.

To guarantee that Americans are protected from potentially unsafe levels of pesticide residues on food, the new law directed the EPA to reexamine the allowable levels for hundreds of different pesticides. Overall, the agency must review about 9,700 levels -- each use of a particular pesticide on an individual crop. The law requires complete evaluations of one-third of them by Tuesday, focusing first on those that pose the highest risk.

Although among the most important and far-reaching provisions in the law are stringent safety requirements for children -- a response to a 1993 National Academy of Sciences report -- the law also requires the EPA to take into account the aggregate risk from different sources -- drinking water and pest-control efforts in the home, for example, as well as food -- and to consider the cumulative effects of pesticides that act in a similar manner.

"The science here is enormously challenging," said a senior EPA official who spoke about the upcoming announcement on the condition of anonymity. "The act requires us for the first time ever to look at all the exposure pathways for these chemicals. . . . All of it is very controversial."

About 60 million pounds of the cheap and effective organophosphates are used on about 60 million acres each year, and another 17 million pounds are used in the home and for other non-agricultural purposes. Organophosphates work by interfering in the normal transmission of nerve impulses, and they are effective against many types of insects.

Though they do not persist in the environment like the banned pesticide DDT, organophosphates as a class are highly toxic, which is why some were produced as nerve agents during World War II.

Alarmed by their potential to harm the developing nervous systems of infants and children -- who eat many foods with organophosphate residues -- environmental groups have called for a ban on many of these chemicals. In a recent report, for example, the Environmental Working Group estimated that more than 1 million children a day consume "an unsafe dose" of organophosphates.

"We've been saying for a long time these organophosphates pose a risk to kids," said Ken Cook, the organization's president.

But despite the attention paid to organophosphates, it is unlikely that when the EPA announces the results of its reassessment Monday it will have completed its work on those chemicals. According to several people who have closely tracked the agency's progress, EPA is likely to take action against a few high-profile targets -- banning methyl parathion after this year's growing season, for example -- while setting a timetable for completing action on the remaining organophosphates.

And in those cases where it does propose to eliminate specific pesticide uses, EPA will establish "a common sense-type transition strategy" to minimize hardships for farmers, said an agency official.

"What they want to do is announce some very modest steps on a handful of organophosphates to suggest they are doing what they should be doing," said Erik Olsen, a senior attorney with the Natural Resources Defense Council, which is planning to file a lawsuit against the EPA over the issue. "But clearly they are not going to have taken action on the vast majority of organophosphates by Aug. 3. They are way behind."

Jay Vroom, president of the American Crop Protection Association, which represents the interests of farmers and the pesticide industry, predicts that the agricultural community will be treated fairly in Monday's decision. "I think the agency has struggled mightily and done good work," he said. "I'd say it will be fair, and there will be some pain, but we will survive."

Olsen and other environmentalists feel that, to this point, the agricultural and chemical industries have won the battle over implementation of the pesticide law. "They've pulled out all the stops," Olsen said.

A key moment, the environmentalists argue, came in April of last year, when Vice President Gore sent a memo to EPA Administrator Carol M. Browner and Agriculture Secretary Dan Glickman. Gore directed that the Department of Agriculture -- long viewed by environmentalists as favoring the pesticide interests of farmers -- be included in "a sound regulatory approach" that would give "due regard" to the "needs of our Nation's agricultural producers."

To the administration's critics in the environmental community, that sent a clear signal to an agency that the Environmental Working Group has called a "farm team for the pesticide lobby" because so many top pesticide regulators have gone on to represent industry interests in the private sector.

If his side is winning the faraway Washington battle, it sometimes doesn't feel that way to California farmer Strong, who worries what he will do if the EPA takes methyl parathion out of his pest control arsenal. "We have no big hammers to replace it," he said.