Anthelmintic activities of fenbendazole were evaluated in a controlled experiment. Forty calves artificially infected with gastrointestinal nematodes were allotted to 4 groups. Calves in group 1 were used as nonmedicated controls; the other calves were given (orally) fenbendazole at the dose level of 5 mg/kg in the forms of a suspension (group 2), medicated feed to individual calves (group 3), and medicated feed to the group (group 4). These treatments were given on day 35 after calves were inoculated with infective nematode larvae. In groups 2, 3, and 4, overall reductions (based on geometric means) were 99.5%, 99.7%, and 99.6%, respectively. These reductions were highly significantly different (P less than 0.01) from the control calves. Nematodes present in the calves were Haemonchus contortus, Ostertagia ostertagi, Trichostrongylus axei, T colubriformis, Cooperia onchophora, C punctata, and Oesophagostomum radiatum.