Efficacy of Arosurf MSF base formulations of 3 commercial preparations of Bacillus thuringiensis var. israelensis (B.t.i) in controlling mosquito larvae, pupae, and emerging adults was determined in bioassay and preliminary field evaluations. Results of tests of the combinations at application rates that were at and below label recommendations for each product indicated that formulations of Arosurf MSF and commercial B.t.i. would produce significantly better general control of mixed developmental stages of mosquitoes than either of the formulation components. Techniques for utilization of the Arosurf MSF-B.t.i. formulations for operational mosquito control are discussed.