The efficacy and minimum therapeutic dose of closantel, an injectable salicylanilide anthelmintic, was evaluated on adult Ancylostoma caninum. Doses of 7.5 and 10 mg/kg had a marked anthelmintic effect on adult stages, removing 99 and 98%, respectively. A second experiment tested the efficacy of closantel against hypobiotic larval stages of A. caninum. A dose of 20 mg/kg did not affect the abundance of arrested hookworm larvae or prevent their subsequent development, but it may have inhibited their full maturation as adult worms so that patent, intestinal infections did not occur. Small numbers of adults were recovered from treated dogs at 29, 43, and 57 days post-treatment, but no eggs were passed in their feces. These studies bear on the practical problem of control of hookworms in dogs.