Union officials say Stillwater Mining Co. has laid off 31 workers.
Sandi Luckey of the Montana AFL-CIO says the workers laid off over the weekend were not members of United Steelworkers local 11-0001.
She says union members were not given notice or explanation for the layoffs.
Stillwater official declined to comment Wednesday before a May 1 call with investors.
The Billings-based company operates palladium and platinum mines in the Beartooth Mountains, the only such mines in the United States.
Shares of Stillwater dropped .04 cents Wednesday to close at $15.38 per share.