The hypothesis that dermally applied imidacloprid may transfer from treated cats, Felis catus L., to their immediate environment in quantities sufficient to have a significant effect on developing immature cat fleas, Ctenocephalides felis (Bouché), was tested in a controlled experiment. Flea eggs harvested from untreated donor cats were incubated on replicated samples from blankets used by treated or untreated cats under standardized conditions. As compared with controls, the percentage of adult flea emergence on blankets used by treated animals was reduced by 100% in the 1st wk after treatment and by 84, 60, and 74% in subsequent weeks (P < 0.001).