An experimental Sandoz formulation of Bacillus thuringiensis var. israelensis (SAN 402 SC 98) was several times more effective than Abbott ABG 6188 against larvae of Aedes aegypti and Ae. vexans in the laboratory. Field applications of SAN 402 SC 98 at 0.25 liter/ha and ABG 6188 at 1.00 liter/ha resulted in more than 97% control of Ae. vexans larvae after 48 hours, with residual activity of 24 hours or less. The amphipod, Hyallela azteca, and 4 species of water beetles were apparently unaffected by 100 ppm SAN 402 SC 98, which is 1,449 times the 24 hour LC50 for third instar Ae. vexans larvae.