state environmental agency charged with protecting Lake George is proposing the first use of an herbicide in an Adirondack lake, to fend off an insidious weed that has choked native plants and mired swimmers and boats.

Under a proposal by the Lake George Park Commission, about 175 pounds of the herbicide, Sonar, would be applied to 36 of the 28,000 acres of Lake George as early as June. Sonar, which contains the active ingredient fluridone, has been used to kill the weed, Eurasian watermilfoil, in New York since 1995, but never in the state-protected Adirondack Park.

The use of Sonar in Lake George, which would be financed with $215,000 in state and private grants and donations, must still be approved by the state's Adirondack Park Agency and the Department of Environmental Conservation. A two-hour public hearing last week in the town of Lake George drew more than 50 people on both sides of the issue.

Michael White, executive director of the Lake George Park Commission, said in an interview that use of the herbicide was necessary to save the lake from the encroaching weeds. "It's out of balance and not going to right itself," he said of the lake.

But some local residents say Sonar would kill rare plants in the lake and pose a health risk to people who swim in the water or drink it.

"It's a chemical quick fix," said Ted Brothers, 72, a Presbyterian minister who has lived on the shores of Lake George for a decade. "And I don't think it can be done safely."

Nina Habib Spencer, a spokeswoman for the federal Environmental Protection Agency, said that Sonar, when used correctly, is considered one of the less toxic pesticides and is not believed to harm humans. The agency has ranked it in the same category of toxicity as malathion and Anvil, used against mosquitoes, and many over-the-counter pesticides.

But several environmental groups have voiced concern that the use of Sonar would undermine their efforts to persuade towns in the Adirondacks to refrain from using pesticides. "We're worried that the idea would be expanded to the other 2,800 lakes and ponds and we'll be fighting it over and over," said John F. Sheehan, a spokesman for the Adirondack Council, an advocacy group.

Mr. Sheehan said that state environmental officials have allowed two types of pesticides to be used in the Adirondacks: rotenone, which kills fish and other aquatic species that have taken over the natural habitat of the brook trout, and aerial sprays that kill black flies.

Eurasian watermilfoil has infiltrated lakes and ponds around the country, including more than a dozen in the Adirondacks. The weed can grow as tall as 20 feet, and has long, feathery branches that form dense mats on the water's surface.

The watermilfoil was identified in Lake George in 1985, and is growing in 136 sites around the lake. Previous efforts to control the weed by pulling it out by hand or with a suction hose, or smothering it with plastic mats, have cost more than $750,000. "It's not enough," said Charles Boylen, a biology professor at Rensselaer Polytechnic Institute who oversees watermilfoil research on Lake George. "There are areas so dense that all we've been able to do is let it grow."

Professor Boylen and others say the watermilfoil has already killed rare plants, and would crowd out other native plants if not reined in.

"We're not a big promoter of the use of chemicals," said Mary-Arthur Beebe, executive director of the Lake George Association, a nonprofit citizens group. "But we think the milfoil is the most dangerous risk to the lake by far."