Experiments were conducted to determine the effects of low temperature (0 and 4 degrees C), vs. a high temperature (22 degrees C), on the feeding rate of Aedes stimulans larvae, and their susceptibility to Bacillus thuringiensis var. israelensis (H-14) (B.t.i.). Third-instar Ae. stimulans slowed but did not halt feeding at 0 and 4 degrees C compared to 22 degrees C. Susceptibility of larvae, as measured by LC50 values, to B.t.i. was highest at 22 degrees C (LC50 = 0.1 ppm), and lower at 4 degrees C (LC50 = 0.2 ppm) and 0 degree C (LC50 = 0.9 ppm). The data from the feeding and susceptibility experiments suggest that decreased efficacy of B.t.i. at low temperatures may occur because the rate of larval feeding decreases. Low water temperature should be a consideration during operational applications of B.t.i. for control of larvae in cold-water habitats, such as the spring Aedes species.