A controlled clinical trial was performed to determine the duration of efficacy of a new oral insecticide formulation of spinosad for the control of experimentally induced Ctenocephalides canis infestations in dogs. Twelve Beagle dogs (two groups of six) were used in the study. Dogs in the treated group received spinosad tablets per os on D0 at the commercial dosage. All dogs were infested with 100 fleas on Days - 7, - 1, 7, 14, 21, 28 and 35. The dogs were combed four hours after each infestation and fleas were counted and replaced on the coat. 24 hours after each infestation fleas were combed, counted and removed. The efficacy of the formulation was calculated four and 24 hours after the treatment and then four and 24 hours after each new infestation. The mean number of fleas on the control dogs was respectively between 65.1 and 83.3 at four hour counts and between 58.3 and 75.3 at 24 hour counts. The product was well tolerated. The treatment controlled the fleas already present on the skin with 81% efficacy at four hours and 100% efficacy at 24 hours. For the weekly infestations, the speed of action of the product was high: at four hours the efficacy was 100% at D7, 96% at D14, 74% at D21, 42% at D28, 12.90% at D35 and 12.8% at D42. The efficacy evaluated 24 hours after each infestation was approximately 100% during three weeks then 90% at D39, 81.4% at D36 and 80.4% at D43. A single dose of the new spinosad tablet formulation should control flea populations in dogs for four weeks as indicated in the claim (evaluation performed at 48 h for the registration). Spinosad tablet is the first product administered per os which acts so long and so quickly against adult fleas.