Poisonous plants are a major cause of economic loss to the livestock industry. Each year these plants adversely affect 3 to 5 percent of the cattle, sheep, and horses that graze western ranges. These losses result from death of livestock, abortions, photosensitization, decreased production, emaciation, and birth defects. In addition to these losses are those of increased management costs associated with such things as fencing, altered grazing programs, and loss of forage. This bulletin describes more than 30 of the principal poisonous plants growing on western ranges and the signs of poisoning in livestock. Suggestions are included for the prevention of livestock poisoning by plants.