The effect of Bacillus sphaericus, at various concentrations, on hatching of phlebotomine sand fly eggs was examined using laboratory bioassays. Aqueous suspensions of B. sphaericus, strain 2362, inhibited hatching of eggs of Phlebotomus duboscqi and Sergentomyia schwetzi by 95% at concentrations as low as 0.05 and 0.11 mg/cm2, respectively. In contrast, B. sphaericus did not affect the ability of pupae to emerge as adults.