An isolate of Bacillus thuringiensis var. darmstadiensis (H-10) with preferential toxicity for mosquitoes was evaluated in the laboratory against Culex quinquefasciatus, Aedes aegypti, Anopheles albimanus and An. quadrimaculatus; the LC50 values for 2nd instar larvae were 0.41, 3.96, 4.62 and 5.38 viable spores X 104 /ml, respectively. There was a strong positive correlation between temperature and larvicidal activity against fourth instar Cx. quinquefasciatus exposed to 3.46 x 104 viable spores/ml at 18, 24 and 31oC. Second instar Cx. quinquefasciatus and Ae. aegypti were approximately 10 x more susceptible to the spore-crystal suspension than were fourth instar larvae.