Laboratory bioassays of Bacillus thuringiensis israelensis (BTI) were conducted against 5 larval age groups of Aedes aegypti and Ae. taeniorhynchus. Ae. aegypti was more susceptible to BTI than Ae. taeniorhynchus in all age groups except the 6 hr old one. For 72 hr old larvae, the LD50 was 0.36 ppm for Ae. aegypti and 0.52 ppm for Ae. taeniorhynchus after a 48 hr exposure to BTI. Younger larvae of both species were more susceptible than older larvae. Comparative assays were run with IPS 78 material from the Institut Pasteur and a portion of the data is expressed as relative activity of facilitate comparison with other studies.