Monsanto, which makes Roundup from the phosphate it mines in Idaho, announced the sheep deaths Friday afternoon.
Company spokesman Trent Clark says the sheep died after eating contaminated plants in the Henry Mine, which was closed in 1989.
Clark says a herder entered the pit with 1,200 sheep without permission.
According to Monsanto, the herder noticed several were getting sick and moved them, but it was too late for 95 animals.
A federal lab at Utah State University confirmed the sheep likely died after ingesting western aster, which accumulates selenium from contaminated soil.