WASHINGTON -- Four former directors of the National Park Service are asking Interior Secretary Gale Norton to reconsider a decision allowing continued use of snowmobiles in Yellowstone and Grand Teton national parks.

"It is our deep hope as this issue now moves to your final review that you will ensure the highest protection for Yellowstone," former directors George Hartzog Jr., Russell Dickenson, Roger Kennedy and Robert Stanton wrote in a letter dated Tuesday. "To do otherwise would be a radical departure from the Interior Department's stewardship mission."

The men served stints in Republican and Democratic administrations since the 1960s and are asking Norton to revive a proposal made during the Clinton administration to ban snowmobiles from the popular Western parks.

David Barna, the chief of public affairs for the Park Service, said the department typically does not comment on letters to Norton until she writes a response.

Beginning this winter, the Park Service will require snowmobilers to get reservations to enter the parks, which sit in northwestern Wyoming and include parts of Montana and Idaho.

The parks also will set daily limits on the number of snowmobiles and noise and emissions standards that will require cleaner and quieter machines, Barna said.

The decision was described as final when it was published in March. But the formal rules it sets out will be published in October, making it possible "in theory" the decision could be pulled back, Barna said.

The service does not now limit the number of snowmobiles allowed in the parks. Under the Clinton administration, however, the Park Service proposed phasing out snowmobiles and only allowing special snowcoaches instead. Officials called that plan the best way to protect wildlife and minimize pollution.

That rule was revisited under the terms of a settlement with snowmobile manufacturers, who sued the government over the proposal. The current plan stemmed from that review.

The former directors maintain the Clinton-era proposal would be cheaper and better for the parks and most visitors. Two past Yellowstone superintendents, a former assistant Interior secretary and a former Park Service deputy director also signed the letter.

Barna said the decision is a balance between the Park Service's responsibility to both protect the park and maintain public access.