During the last decade, federal agencies working with the state of Montana captured and shipped to slaughter more than 3,300 bison to prevent the spread of an animal disease to cattle.
Critics of that program have tried unsuccessfully to stop the practice through lobbying of government agencies and appeals to Congress.
Now they're asking a U.S. District Court judge to intervene. The lawsuit filed Monday would bar the National Park Service and Forest Service from participation in the slaughter program.
The plaintiffs contend the two federal agencies have become beholden to the cattle industry and are ignoring their wildlife conservation mission. They also say the threat of the disease, brucellosis, has been overstated.
"It's crazy for me to think that in a state like Montana, where we are rich in wildlife and wildlands, that we don't have room for bison," said Tom Woodbury with the Western Watersheds Project.
Woodbury's group is one of nine plaintiffs in the case, which will likely be heard by Judge Donald Molloy in Missoula.
Bison once roamed North America by the millions before being largely wiped out in the late 1800s. Yellowstone's 3,000 bison comprise one of the largest concentrations of the animals remaining in the world.
About half those animals test positive for exposure to brucellosis, a reproductive disease that causes pregnant animals including cattle to abort their young.
During severe winters and when bison numbers are high, thousands of bison attempt to migrate to feeding grounds outside Yellowstone. But under a 2000 agreement between Montana and the federal government, the animals can be killed to prevent any contact with cattle.
In 2008, more than 1,400 bison were captured and shipped to slaughter under that policy, cutting the park's population by more than a third.
Soon after, the Government Accountability Office released a scathing report admonishing Montana and federal agencies for failing to preserve Yellowstone's bison.
As a result, government agencies promised to be more flexible. They allowed bison to leave the park for some areas where cattle no longer graze. And a migration corridor was leased through a private ranch adjacent to Yellowstone to let at least 25 bison access Forest Service land outside the park.
Representatives of the cattle industry fought both moves.
Forest Service spokeswoman Marna Daley said the agencies were "looking for opportunities for bison to move outside of Yellowstone National Park without increasing the risk of the spread of brucellosis."
However, she acknowledged the process had moved slowly, and said the agencies were forced to take "baby steps" due to pressure from both sides on the issue.
Bison control used to be carried out largely by the Montana Department of Livestock. Under the 2000 agreement, management of the animals has been coordinated by a group of five federal and state agencies, including the Forest and Park services.
What additional steps might be taken by those agencies is unclear. Yellowstone spokesman Al Nash said the park is committed to a study looking at the potential for vaccinating bison against brucellosis, but could not offer a timeline.
The groups behind Monday's lawsuit dismissed steps taken to date as bureaucratic dithering.
"They are slaughtering these animals indiscriminately. We might be seeing the slow extinction of the American bison," said Stephany Seay with the Buffalo Field Campaign, another plaintiff in the case.
Brucellosis first came into the Yellowstone region through the cattle of European settlers. It has since been eliminated in the livestock industry and is found only in Yellowstone's wildlife.
Elk also carry brucellosis and are considered the likely culprits in at least seven transmissions to cattle over the last decade.
However, the Yellowstone region's estimated 100,000 elk are not subject to a slaughter policy. That's largely because they are more difficult to track and are found over a far wider range.