This study was conducted to determine the susceptibility of free-ranging black-tailed deer (Odocoileus hemionus columbianus) to tansy ragwort poisoning. The results showed that captive black-tailed deer would not consume tansy ragwort unless they were unable to acquire adequate intake from other food sources. Since some experimental deer consumed an average of 24% of their body weight as dry matter from tansy ragwort over a 42-day period without any symptoms of toxicity, it appears that deer may be relatively tolerant to poisoning by this plant. It seems unlikely that tansy ragwort would cause toxicity problems with free-ranging black-tailed deer.