In an effort to control Protostrongylus spp. in a Rocky Mountain bighorn sheep herd (Ovis canadensis canadensis) of approximately 30 animals, fenbendazole-medicated salt was placed on the Stillwater bighorn winter range in southcentral Montana (USA) for four consecutive winters, 1990 to 1993. Sheep of all age and sex classes were observed using the medicated salt throughout the study period. Prevalence and average number of lungworm larvae per gram of bighorn feces declined significantly (P < 0.05) from pretreatment levels (1987 to 1989), and remained low throughout the study period. Free-choice availability of fenbendazole-medicated salt is a potentially effective management tool for long-term control of protostrongylid lungworm.