Two wettable powder formulations of Bacillus thuringiensis var. israelensis (B.t.i.) manufactured by Abbott Laboratories and Biochem Products were tested for efficacy against 2 species of mosquito larvae in simulated pool conditions. Their effect on some aquatic insect predators of mosquito larvae was also investigated. Both formulations proved effective in controlling the test mosquito species. The Biochem Products material was formulated at a higher potency and gave 100% control of 4th instar Culex restuans after 12 hr at 0.8 mg/; the Abbott formulation gave 25% control after 12 hr. When applied at 3.2 mg/e, the Abbott formulation gave 100% control of the 4th instar Aedes vexans after 24 hr. Second instar larvae of both species were generally more susceptible than 4th instar larvae. Effective residual activity was less than 24 hr at the highest rates tested for both the Abbott and Biochem formulations. Mortality was negligible in non-target invertebrate predators following a 5 day exposure to 12 mg/l of both formulations of B.t.i.