Environmentalists took the National Park Service to court yesterday over ongoing delays in implementing a ban on snowmobiles in Yellowstone and Grand Teton national parks. In 2000, the agency approved a plan that would have limited the number of snowmobiles in the parks to under 500 by this winter and mandated that drivers be accompanied by park guides. But the agency has yet to implement the plan, saying they are developing a different one instead. The lawsuit also challenges the federal government’s failure to respond to a 1999 petition by the environmental group Bluewater Network to ban snowmobiles in all national parks. The suit is not expected to have any immediate effect on snowmobile use, and at present the Park Service is planning to allow an unlimited number of the vehicles in Yellowstone and Grand Teton this winter.