Anthelmintic activity of albendazole against adult Dictyocaulus viviparus was evaluated in a controlled experiment. Calves were raised nematode-free to approximately 8 weeks of age and were each given 4,000 infective 3rd-stage larvae. Twenty calves with patent parasitisms were allotted to 2 groups of 10 calves each. Calves in group 1 were used as nonmedicated controls, and calves in group 2 were given albendazole in paste formulation at the dosage concentration of 7.5 mg/kg of body weight on the 30th day after administration of infective larvae. At necropsy, nonmedicated control calves had a total of 308 adult D viviparus, whereas the albendazole-treated calves had 11, for an average efficacy of 96.4%. These reductions were statistically highly significant (P less than 0.01). At necropsy, none of the treated calves was passing 1st-stage C viviparus larvae in their feces, whereas control calves were passing an average of 46 larvae/10 g of feces.