WASHINGTON (AP) _ The Interior Department reached agreement Thursday with a major Florida sugar company on a long-term cleanup plan aimed at preserving the ecological health of the Everglades.

The agreement with Flo-Sun Inc. ''will ensure that the company meets its responsibilities to fund Everglades pollution cleanup over the next 20 years,'' the department said in a statement.

The development is a major step in the Clinton administration's effort to require Florida sugar growers to reduce runoff pollution that has contaminated the Everglades river system and wetlands.

A negotiated settlement between the government and the growers collapsed last month when some growers demanded that Interior promise not to impose certain Everglades restoration projects.

The agreement with Flo-Sun represents a clear split among the growers.

U.S. Sugar, another major grower, has refused to sign the agreement, the department said. The two companies each control about 30 percent of the agricultural lands in the Everglades region.

The decision by Flo-Sun represents a major step forward in crafting a protection plan for the vast wetlands area, said Assistant Interior Secretary George Frampton. ''It's proof that the industry can become partners with the federal government.''

Noting that U.S. Sugar remains opposed to the government's cleanup plan, Frampton vowed to ''redouble our litigation and regulatory efforts against those who are unwilling to join in such an agreement.''

He said the administration also would press for ''prompt, fair and effective environmental restoration efforts'' in the Everglades.

Under the agreement with Flo-Sun, the sugar grower has agreed to share in payments for reducing runoff pollution, increase the on-farm treatment of runoff water to reduce phosphorous levels, and agreed to construction of a regional water treatment system.

Environmentalists for years have sought to curb the high levels of phosphorus in the Everglades waterways, blaming high-phosphorous runoffs from the sugar growing farms. They say the phosphorous has choked the ecosystem threatening one of the countries most important ecological treasures.

''This agreement strengthens the position of those who seek to clean up Everglades water as soon as possible by securing new funding for cleanup and at the same time putting additional litigation cost burdens on those who choose to delay the Everglades cleanup process in the courts,'' said Frampton.

As part of the settlement, Flo-Sun also agreed to stop paying into an industry-wide fund used to pay for legal fees to challenge the government proposed cleanup plan.