Death, poor condition and unsatisfactory breeding performance in a rabbit colony was attributed to infection with Passalurus ambiguus. The trouble disappeared after treatment with 50 ppm fenbendazole in the food for 5 days. In laboratory trials 12.5 ppm for 5 days eliminated more than 99% of adult and most immature pinworms, 25 and 50 ppm eliminated all immature and adult pinworms. These doses were also effective against Obeliscoides cuniculi.