One of the fiercest, if least known, battles currently under way in Washington these days boils down to these epic dimensions: billions of angry little red ants versus nine angry Southern state agriculture commissioners.

On the surface, at least, the issue is a relatively simple one. The Environmental protection Agency must decide soon whether to approve a newly developed pesticide called ferriamicide that would be used to kill that Southern scourge, the stinging fire ant.

But behind the scenes is a struggle tangled enough to make Faulkner's ghost blink. Involved are one very powerful southern senator, a major environmental bill, $20 million in federal and state agriculture subsidies, some very deeply entrenched political power bases throughout the Deep South and perhaps even the Fourth of July as we now know it.

The ferriamicide decision arose out of an EPA ruling two years ago that mirex, another potent pesticide, could no longer be used against fire ants because it was a suspected carcinogen. Mirex not only produced tumors in laboratory animals, the EPA said, but it broke down in the ground to form kepone, another contaminant banned by the EPA.

The ruling left the South without an effective weapon to kill the fire ant. Southerners have felt the wrath of the tiny but aggressive past since 1918 when it was accidentally introduced into Mobile, Ala., by an elderly bug expert. Since then it has made its way through nine southern states and over 190 million acres, ruining picnics and torturing farmers and fieldhands along the way.

After the EPA decision, the Allied Chemical Co., the major mirex producer, shut down its Mississippi manufacturing plant and sold it to the state for $1. Mississippi, the heaviest user, made the pesticide itself for a while, then shut down and started looking for a substitute.

Last month Mississippi announced it had developed ferriamicide, which it said was mirex with chemical amines added to make it break down so fast and thoroughly that nothing harmful was left. The state enlisted Sen. James O. Eastland, the powerful Mississippi Democrat, to make its case for an emergency approval from EPA to spray ferriamicide and sell it to other southern states.

Meanwhile the environmental Defense Fund, which led the fight against mirex, filed an objection to any quick approval by EPA for the pesticide. The environmental group said ferriamicide was still mirex-based and not enough was known about the new combination to prove it wasn't dangerous.

The fight was on. Letters poured in - 12,000 in the last month to the EPA from irate Southern farmers - the phones at the regulatory agency began ringing off the hook, including more than 15 calls from southern congressmen supporting approval of the pesticide. Eastland, whose staff engineered much of the ferriamicide campaign, requested an audience on the subject this week with another southern farmer - Jimmy Carter.

There are also fears among EPA officials and other s that if Eastland does not get his way with ferriamicide he could delay or kill final approval of a major revision in the Federal Insecticide, Fungicide and Rodenticide Act. The act is a pet EPA project which is now in a conference committee that includes Eastland and several other powerful southerners. Eastland could not be reached for comment.

Carter, who opposed mirex while he was governor of Georgia, had not met with Eastland by yesterday. But a shaken senior EPA official said the campaign for the pesticide was the most intense to hit the agency in years.

There may be more than fire ants behind the pesticide pitch, however, according to knowledgeable federal officials. At stake, they said, is $20 million in matching federal and state funds which go to the southern state agriculture commissioners each year for fire ant control. Several of the commissioners were in Washington last week helping coordinate the pro-pesticide campaign.

Without the federal pesticide approval, the money might be lost and with it, the officials noted, a powerful political weapon for the southern commissioners, many of whom hold unusual political power in the rural South.

"Forget the ants," one federal official said. "The key is the money. It lets these guys make jobs, launch fleets of spray planes and get up at the Fourth of July picnics and tell everyone that with out us to get rid of those ants you would not be here."