Preliminary Studies showed spinosad to be rapidly effective and safe in controlling fleas on dogs. To validate these studies, a clinical trial was undertaken using 470 flea-infested client-owned dogs allocated to receive three monthy treatments with either beef-flavored chewable spinosad tablets (30-60 mg/kg) or selamectin applied to label instructions. Flea counts 15 days after enrollment were reduced by 98.6% and 90.9% for spinosad- and selamectin-treated dogs, respectively; at 90 days, flea count reductions were 99.9% and 98.9%, respectively. Compared with baseline, all flea reductions were significant (P less than .001) for both products and spinosad was significantly (P less than or equal to .0172) more effective than selamectin at each postenrollment flea count.