One controlled and six critical tests were conducted in July, 1977 with northern fur seal pups (Callorhinus ursinus) to determine the efficacies of a single dose of dichlorvos capsules at 29.3 to 32.8 mg/kg, tablets at 10.5 to 11.5 mg/kg, or disophenol at 9.9 mg/kg given subcutaneously against natural infections of adult Uncinaria lucasi. In the controlled test, 20 pups were treated and 10 pups were nontreated. Removal of hookworms in this test was 99% for five pups receiving dichlorvos capsules, 99% for five pups receiving dichlorvos tablets, and 77% for 10 pups receiving disophenol. Also, both formulations of dichlorvos and the formulation of disophenol were highly active against natural infestations of two species of sucking lice (Proechinophthirus fluctus and Antarctopthirus callorhini). In critical tests with four pups treated with dichlorvos capsules at 28.6 to 30.6 mg/kg, removal of hookworms was uniformly 100%. Disophenol at 9.9 mg/kg removed 100% and less than 1% of hookworms in two pups, respectively, in critical tests.