In the aftermath of two court rulings rejecting rulemakings on winter use plans for Yellowstone, the National Park Service is once again being challenged in court for failing to consider the effects on bison populations of winter use plans that accommodate snowmobile use. Although media attention has focused on the Clinton administration's ban on snowmobiles in the Yellowstone area parks and the Bush administration's rollback of that ban, the initial impetus for the snowmobile rulemakings was the effect on wildlife of "trail grooming." Trail grooming is the practice of packing 25-foot wide paths, in essence creating roadways for snowmobiles. Although a variety of Yellowstone wildlife has experienced adverse consequences of trail grooming, the most dramatic consequence is that bison are attracted to the groomed trails and leave the protected park areas, making them vulnerable to slaughter. After more than 1,100 bison were slaughtered in the winter of 1996-97, the Fund for Animals and other environmental groups challenged the Yellowstone winter use plans under the National Environmental Policy Act. The NPS settled with the groups, agreeing to prepare an environmental impact statement on trail grooming practices and the related policy of allowing snowmobiles into the parks. Subsequent rulemakings -- a Clinton administration decision to ban snowmobiles in favor of snowcoaches, and a Bush administration rollback of that ban -- were subject to court challenges described here. Among other outcomes, a federal court ruled that the NPS had failed to address adequately the effects of trail grooming on bison migration patterns. The NPS prepared a new environmental assessment and announced a continued preference for trail grooming and limited snowmobile access. Even though a scientific assessment of trail grooming's effects on bison will not be available until March 2005, the NPS declared a finding of no significant impact (FONSI) that led to a final rule on winter use in the parks that permits continued trail grooming. Alarmed that the new winter use policy still fails to address problems of trail grooming, the Fund for Animals filed suit on Nov. 4 challenging the new winter use plan as violating NEPA, the APA, the Organic Act, the Yellowstone Act, and previous court orders.