The efficacy of selamectin was evaluated against naturally acquired Trichodectes canis infestations on dogs and against Felicola subrostratus infestations on cats. Twenty dogs and 18 cats were randomly allocated to treatment with either a placebo or selamectin (6 mg/kg), administered topically once only on day 0. The treatment had no adverse effects in either the dogs or the cats. Efficacy was assessed by counting the live lice (adults and nymphs) on each animal by using a coat.parting technique on days -3, 7, 14, 21, 28, 35 and 42 for the dogs, and on days -1, 7,21, 35 and 42 for the cats. On day 43, the number of live lice on each dog was also assessed by using a whole.body combing technique. Selamectin was 100 per cent effective in killing biting lice on the dogs and cats throughout the period of assessment; the louse counts on the treated dogs and cats were significantly lower than the pretreatment counts (P=0.0001) and were also significantly lower than on the placebo.treated dogs (P≤0.05) and cats (P=0.0001). There was a marked reduction in the prevalence of clinical signs associated with ectoparasite infestation in the treated dogs and no clinical signs were observed in any of the treated cats.