Just hours before the first snowmobiles of the season were to rumble through Yellowstone National Park, a judge left park officials scrambling to comply with Clinton-era entry rules that the Bush administration had scrapped.

The decision, issued late Tuesday by a judge in Washington, D.C., cut sharply the level of snowmobiles allowed to enter Yellowstone (search) and Grand Teton (search) national parks each day in order to reduce pollution.

The ruling won't immediately close the parks to snowmobiling. A limited number of snowmobiles will be allowed this winter — but all must be part of commercially-guided trips.

Under the new rules, 493 snowmobiles (search) are permitted each day in Yellowstone and 50 each day in Grand Teton and on the parkway that connects the parks. By the winter of 2004-05, only mass-transit snow coaches would be allowed.

Wyoming Attorney General Pat Crank said he planned to file an appeal of the decision Wednesday, asking that Yellowstone be allowed to operate under the Bush administration rules that permit more snowmobiles and no supervision this winter.

"We think the rule adopted by the Park Service is the correct rule," Crank said. "It balances the ecological concerns of Yellowstone with regard to wildlife, with folks being able to use Yellowstone National Park during the winter season."

The scramble to comply was sparked when U.S. District Judge Emmet Sullivan ruled that the Bush administration should not have set aside the Clinton-era plan.

Under Bush, the Park Service opted to let snowmobiling continue but to cap the number of snowmobiles allowed and only permit cleaner, quieter snowmobiles. The plan would have allowed a total of 1,140 snowmobiles a day for the parks and parkway.

In a lawsuit, the Greater Yellowstone Coalition (search) of Bozeman claimed the Park Service ignored its own studies showing a ban on snowmobiles and the use of snowcoaches would best protect park resources.

The group said unacceptable pollution and health risks to workers would have continued even with the new emission and entry limits on snowmobiles. Some workers at Yellowstone's popular west gate have worn masks, particularly on busy days when there was congestion at the entrance.

The Park Service argued that the Bush plan was based on cleaner snowmobile engines that weren't considered when the earlier ban was drafted. But Sullivan rejected the argument.

"The prospect of new technology is not 'new,'" the judge wrote, noting that less-polluting machines were considered and rejected when the Clinton administration was deciding how to reduce the harmful effects of snowmobiling.

Interior Secretary Gale Norton defended the Bush plan late Tuesday, calling it "a responsible approach" that avoids a complete ban on snowmobiling. "Allowing the limited use of snowmobiles as a means of access makes possible a wider array of enjoyment opportunities," she said.

Conservationists and others praised the return to Clinton-era rules.

"Yellowstone is where our country first said, 'This is what our national parks mean to us.' This ruling reaffirms that fundamental purpose," said Denis P. Galvin, a former deputy director of the Park Service.

But some local businessmen were upset by the ruling.

David McCray, who has a snowmobile rental company in West Yellowstone, said he had invested heavily in the cleaner snowmobiles because he thought his business needed them. But he said he never would have done so if the case had been decided earlier this fall.

"For the judge to throw this out at the 12th hour is unconscionable," he said.

The change in rules means visitors who had booked advance reservations to ride in Yellowstone and Grand Teton on their own will now have to cancel since all snowmobiles entering the parks will need to be a part of commercially-guided trips, Yellowstone Superintendent Suzanne Lewis said.