Some strains of Bacillus sphaericus have a high level of insecticidal activity toward larvae of many mosquitoes species. Spores of these strains kill their host by means of a toxin, and recycling can occur in the dead host with net increase in numbers of spores. This toxin affects the larval midgut within 30-60 min, but 48 hr are required for all mortality to be expressed. Although the activity of B. sphaericus spores is apparently degraded by sunlight, this effect is not due to the sterilizing ultraviolet component, Antibiotic resistance of B. sphaericus has been exploited in the production of selective media useful in recovery of this organism from the aquatic environment. Bacillus sphaericus is highly insecticidal to some species of Culex and Anopheles, but is much less insecticidal to certain Aedes species; it is not toxic to black flies. Recent genetic studies of this organism may result in improvement of its host range.