Methyl-5(6)-phenylsulfinyl-2-benzimidazole carbamate (Oxfendazole) (Syntex Research, Syntex (U.S.A.), Inc., Palo Alto, Calif. 94304) a new potent anthelminthic, was tested at nine dosage levels against gastrointestinal and lungworm helminths in naturally infected lambs. A modified least significance difference test ( P= 0.05) was used on log transformed data to test for differences in the average efficacies of the various dose groups as the primary basis for selecting the optimum field dosage level. Five (5.0) mg/kg provided high (⩾90%) efficacy against parasitic stages of the following helminths: Chabertia ovina, Cooperia spp., Dictyocaulus filaria, Haemonchus contortus, Moniezia sp., Nematodirus spp., Oesophagostomum venulosum, Ostertagia spp., Trichostrongylus spp. and Trichuris ovis. Limited numbers of Bunostomum trigonocephalum and Capillaria spp. precluded definitive conclusions regarding efficacy against these parasites. Efficacy against Strongyloides papillosus was erratic at all dosage levels. Toxicity attributable to Oxfendazole was not observed.