The Chesapeake Bay Foundation will file a lawsuit against the Environmental Protection Agency today, signaling a more aggressive strategy for a group that has grown increasingly frustrated over the slow progress in cleaning up the bay.

The lawsuit, a copy of which was released yesterday, accuses the federal agency of dragging its feet as sewage plants across the region pollute the bay's waters. Specifically, the suit asks that the EPA respond formally to the bay foundation petition submitted in December seeking limits on the amount of nitrogen released from such plants.

"Our only recourse is to litigate," said William C. Baker, the bay foundation's president. "We've tried . . . to give EPA a chance to fulfill its promises."

The suit is one of several recent legal steps initiated by environmental groups, which traditionally have relied on consensus, rather than confrontation, with government agencies in the effort to clean up the bay.

The foundation already is suing Virginia over what it believes were lax pollution permits issued for a sewage plant in Onancock and a Philip Morris industrial plant. And Maryland watermen are considering a class-action lawsuit against polluters, including sewage treatment plants and manufacturers that dump into the bay's watershed.

The key issue in the legal action to be filed today is nitrogen, which along with phosphorus serves as food for huge algae blooms in the bay. Scientists blame these algae for creating "bad water" where fish and crabs cannot breathe.

For those trying to clean the bay, the problem is that nitrogen comes from everywhere, including from animal manure that washes off farm fields and from leaky septic systems. The more than 350 sewage plants in the Chesapeake Bay watershed account for about 21 percent of its nitrogen, according to the EPA.

In its petition filed in December, the bay foundation noted that almost none of those sewage plants had permits that limited nitrogen pollution and asked the EPA to remedy that.

EPA regulators responded with a draft proposal that eventually would impose such permits on most of the plants. That plan, if authorized by EPA leaders, could be implemented by spring, agency officials said yesterday.

"There's no disagreement with the bay foundation that this is a necessary next step," said Jon Capacasa, an official at the EPA regional office that oversees the bay.

But the federal agency has not provided a formal answer to the bay foundation's demands, and Baker said the draft proposal contains "weasel words" that might allow the EPA to back out of key provisions.

So the bay foundation decided to go to court, asking a federal judge in the District to speed up the EPA's formal response.

Baker said that environmentalists are frustrated that the bay has made so little progress in the three decades since the cleanup began, and particularly in the four years since the signing of the landmark Chesapeake 2000 agreement.

That agreement -- signed by the EPA, the D.C. mayor and the governors of Maryland, Virginia and Pennsylvania -- set a goal of removing the Chesapeake from the national "impaired waters" list by 2010. With nearly half the time gone, most of the efforts are still in the planning stages, and Baker said his organization has lost patience with the EPA.

"We think that 41/2 years is enough time to determine that they are not interested" in moving more quickly, he said.

Still, some observers, including Bill Matuszeski, who formerly oversaw the EPA's Chesapeake cleanup effort, worry that lawsuits might bog down the process further. "Tying this whole issue up in court isn't going to move it forward at all," he said.

Bay experts say that farmers who spread manure on their fields as fertilizer contribute to bay pollution because of the nitrogen content of the runoff that makes its way to the Chesapeake.William Baker said the foundation's "only recourse is to litigate."