|Abstract||Twenty weaner calves were ranked on the basis of naturally acquired Linognathus vituli and Bovicola (Damalinia) bovis infestations and allocated to one of two equal groups. Calves in one group received doramectin (200 mcg/kg) by subcutaneous injection near the base of the ear while those in the second group were given 0.9% sterile saline by the same route. Louse counts at 7 sites replicated on each side of the animals were carried out at the beginning of the trial and then at 7, 14, 21, 28, and 42 days post-treatment. By 7 days post-treatment the Linognathus vituli infestations were absent from all treated calves and all but 1 control animal. Up to Day 21, all treated animals remained louse-free. The doramectin-treated animals showed significant (P < 0.05) reductions in Bovicola (Damalinia) bovis infestations from 7 to 42 days post- treatment. Efficacies of 81.9, 98.6, 94.2, 99.3, and 95.4% were obtained for Days 7, 14, 21, 28, and 42 respectively, when comparisons were made with non-treated animals for the same days.