Laboratory and field comparisons were made with 5 formulations of Bacillus thuringiensis var. israelensis de Barjac (Bti) against second-stage larvae of Aedes aegypti. The activity (International toxic units per mg) of the test materials was: Abbott Laboratories ABG 6108 WP (6406-125), 651 ITU, ABG 6108 WP (6478-199), 567 ITU, and ABG 6109 (6406-122), 286 ITU ; Biochem Products' 666 PM 50, 3482 ITU; and Sandoz Corporation's SAN 402 I WDC, 688 ITU. Small plot field studies revealed that Bti at 0.7-5.2 kg/ha provided complete control of Ae. aegypti depending upon formulation. Application of 1, 1.5, and 4 kg/ha of SAN 402 I WDC, Biochem 666 PM 50, and ABG 6108 WP (6478-199), respectively, provided complete control of caged larvae of Psorophora columbiae in rice field studies in Arkansas; higher levels were required for Anopheles quadrimaculatus. ABG 6108 WP (6406-125), 651 ITU, ABG 6108 WP (6478-199), 567 ITU, and ABG 6109 (6406-122), 286 ITU; Biochem Products' 666 PM 50, 3482 ITU; and Sandoz Corporation's SAN 402 I WDC, 688 ITU. Small plot field studies revealed that Bti at 0.7-5.2 kg/ha provided complete control of Ae. aegypti depending upon formulation. Application of 1, 1.5, and 4 kg/ha of SAN 402 I WDC, Biochem 666 PM 50, and ABG 6108 WP (6478-199), respectively, provided complete control of caged larvae of Psorophora columbiae in rice field studies in Arkansas; higher levels were required for Anopheles quadrimaculatus. ABG 6108 WP (6406-125) gave 99% and 100% control of Aedes taeniorhynchus in Florida at 2.5 and 5 kg/ha, respectively. In some cases the relative activity of the Bti formulations was reversed or the differences were narrowed under field conditions compared to the laboratory results.