HELENA, Mont. (AP) _ A judge Thursday gave an animal-rights group permission to sue state and federal governments to stop the killing of bison that wander outside Yellowstone National Park.

The judge rejected arguments by the state of Montana and the National Park Service that the Fund for Animals suffered no actual harm from a policy under which buffalo are killed to prevent them from spreading disease to Montana cattle.

The ruling by U.S. District Judge Charles Lovell came after Wayne Pacelle, national director of the fund, argued that the ''extermination effort outside the park'' has caused psychological injury to people across the country.

''No blood has been drained from any of our members in this particular case,'' Pacelle testified. ''The damage is mental and psychological.''

The judge said the New York-based fund's pursuit of animal-rights protections since the 1960s and its previous attempts in his court to stop the slaying of bison gave it legal standing to pursue its case.

The group sued last month for an injunction to stop the bison killing, which it contends violates state and federal environmental laws.

Dirk Van Vuren, an assistant wildlife biology professor at the University of California, Davis, testified after the ruling that Yellowstone's bison are more than simply another herd of buffalo, which are raised domestically in several areas in the West.

Bison killed in Montana on Yellowstone's northern and western borders are direct descendants of only 20 bison that sought refuge in the park and survived the 19th century annihilation of most of their species, he said.

''Bison as a wild animal are extremely rare,'' Van Vuren testified. ''Most other animals in the species are managed, culled and herded.''

Valid research data on bison can be gathered only from buffalo that are free-roaming and not subject to man's manipulation, he said.

An estimated 700 bison have been killed outside Yellowstone during the last five years, and 11 have been killed this winter.

The hunt was ordered by the Montana Legislature in 1985 because Yellowstone's buffalo are diseased. About half the estimated 3,000 bison in Yellowstone are believed to carry brucellosis, a bacterial disease that causes cattle to abort their calves.

Montana cattle herds are certified brucellosis-free. The state contends it must keep the Yellowstone bison from spreading the disease to state cattle herds when the buffalo move in winter from the high Yellowstone plateau area to the sheltered valleys below, where cattle also graze.

Animal-rights activists say the threat is overstated. They question whether brucellosis can be transmitted from bison to cattle, and point out that domestic cattle only graze on the park's boundaries in summer, after the bison have returned to the park.