The efficacy of fenbendazole (methyl-5-(phenythio)-2-benzimidazole-carbamate) in cattle parasitized by the lungworm Dictyocaulus viviparus and in other cattle parasitized by Haemonchus, Ostertagia, Trichostrongylus, Cooperia, and Oesophagostomum was investigated in 3 controlled critical experiments. A single treatment with 5.0 mg/kg of body weight administered in the feed or in a 10% suspension exceeded 99% efficacy against these helminths. Dosages of 1, 0.5, 0.35, and 0.25 mg/kg of body weight administered for 5 days to individual calves in their feed were 99% efficacious against all helminths, except Haemonchus. The 1.0 mg/kg dose removed 99.5% of the Haemonchus, and the other 3 dosages had efficacies of 91.0, 93.7, and 95.6%, respectively. The small daily dosages were not evaluated against Dictyocaulus.