Several strains of the microbial control agent Bacillus sphaericus were evaluated in the laboratory against 2nd and 4th-instar larvae of 5 species of mosquitoes. All strains tested exhibited high levels of activity against Culex larvae. Strains 2362 (IF-97), 2013-4 and 2301-6 were as effective as the standard preparation strain 1593-4 (RB-80), producing 90% mortality in 4th-instar larvae of Cx. quinquefasciatus at a concentration in the range of 0.04-0.05 mg/liter. Strain 1593 (IF-94) was somewhat less active (2 Fold) than the other 3 strains with an LC90 of 0.11 mg/liter. All strains tested displayed lower activity against Anopheles larvae while Ae. aegypti was the least susceptible of all species tested with an LC90 higher than 40 mg/liter. It was demonstrated the 2nd-instar larvae in general were more susceptible than 4th-instars, and that maximum mortality at a given concentration was obtained when an exposure period of 48 hr was used. Unlike B. thuringiensis (H-14) bioassays, the extended exposure period is needed rather than 24 hr because B. sphaericus strains cause little or no mortality during the 24 hr period. Under natural field conditions, both strains of B. sphaericus 2362 (IF-97) and 1593 (IF-94) yielded excellent initial control of Cx. tarsalis and Cx. peus at the rates of 0.1 and 0.2 IB/acre of the primary powders. In some of the field tests, larval control persisted up to 14 days posttreatment. At both rates, these 2 strains had no noticeable adverse effects on prevailing macroinvertebrate fauna such as mayfly naiads, diving beetle larvae and adults, ostracods and conchostracans.