A clinical field trial was performed to evaluate the efficacy of imidacloprid, the active ingredient of Advantage 10% Spot-On, against natural lice infestations on dogs. Imidacloprid was highly effective against both sucking (Linognathus setosus) and biting (Trichodectes canis) lice over the 6-week period of the trial. The duration of efficacy exceeded the egg-to-imago development period of both lice. Signs of remission of dermatitis and alopecia became visible to the pet owners at 2 weeks after treatment. The constant pruritus caused by the lice was seen for up to 2-4 weeks after treatment even in the absence of lice.