Spinosad, a naturally occurring product of the fermentation of the bacterium Saccharopolyspora spinosa, is a highly effective bioinsecticide against a broad range of agriculturally important insect pests, and this agent has an excellent environmental and mammalian toxicological profile. In this study the efficacy of a Spinosad-based product (Laser® 4.8% emulsifiable concentrate) was evaluated in laboratory bioassays against laboratory-reared mosquito strains of 3 species of medical importance: Aedes aegypti, Anopheles stephensi, and Culex pipiens. Spinosad was particularly effective against larval Aedes and Culex, with a less marked activity against anophelines (24-h median lethal concentration = 0.0096, 0.0064, and 0.039 mg/liter, respectively), showing a persistence of the insecticide action of about 6 wk in laboratory containers. The activity of the Spinosad-based product against adult mosquitoes (toxicity by ingestion and a possible irritant or repellent effect on gravid females) also was evaluated. Results are discussed and compared with those available in the literature.