The Big Cypress National Preserve in Florida is crisscrossed with so many illegal swamp-buggy ruts - more than 23,000 miles of them - that park officials in August began limiting off-road vehicles to 400 miles of trails in order to protect the Florida panther and the preserve.

In Yosemite National Park, off-road vehicles are involved in "numerous" violations, according to a staffer's memo.

And each day of a long summer weekend like Labor Day, as many as 2,200 motorized vehicles hit the beaches of Cape Hatteras National Seashore in North Carolina. "Not one of those is there legally," says Don Barger, southeastern regional director of the National Parks Conservation Association, a watchdog group.

In all, unauthorized off-road vehicles are buzzing through nearly one-third of America's national parks, according to a recently released internal National Park Service (NPS) survey. In one-fifth of the parks, they have damaged natural environments that by law must be preserved for future generations.

The survey highlights a conundrum for the park service. It has gone to bat for broader access for motorized off-road vehicles - including snowmobiles in Yellowstone National Park. At least 29 of its parks have allowed illegal off-road vehicles. On the other hand, in some cases, the parks are prohibited from doing so by their authorizing legislation; in other cases, they haven't conducted required environmental impact evaluations of the vehicles, according to a legal analysis by Bluewater Network, an environmental group in San Francisco.

In March, the NPS convened a workshop of rangers and other park officials to develop a "national strategy" for dealing with the problem, according to a senior park official.

NPS officials downplay the challenge.

"Some of the illegal use is very infrequent. Some of it is crossing boundaries that may not be well marked," says Jerry Case, the service's regulations program manager. "We're certainly working on it. [But] there are probably just a handful [of parks] that have significant illegal use causing significant environmental damage."

The Cape Hatteras seashore is one facility that until recently had not conducted the required legal reviews to allow off-road vehicles, though it is doing so now.

"We certainly recognize that some [facilities] need to get back in line with the executive order and regulations, but for the most part I think we're doing all right," Mr. Case says.

Other branches of the Department of Interior, however, say illegal motorized vehicles are a serious national problem. The chief of the National Forest Service, for instance, has called it one of his top four management problems.

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