State officials did not disclose the name of the ranch where the disease was found.
Montana livestock officials announced that a Park County cattle herd has been quarantined after six of the animals tested positive for brucellosis.
Additional testing on the animals was underway to confirm the disease, says State Veterinarian Marty Zaluski.
Introduced by early European settlers and once widespread, brucellosis has since been largely eradicated from cattle nationwide after decades of government effort. It persists in parts of Texas and in wildlife in and around Yellowstone National Park.
Brucellosis can cause pregnant animals to miscarry and has been blamed for weight loss and other problems in cattle. It is not considered a significant threat to human health.
Errol Rice, with the Montana Stockgrowers Association, says the suspected infection in the 150-head herd demonstrates that more needs to be done to eliminate the disease.