The insecticidal activity after field exposure of an aluminum-carboxymethylcellulose microencapsulated formulation of Bacillus thuringiensis israelensis (Bti) spore-toxin complex, with malachite green as photoprotective agent, was evaluated using third-instar Aedes aegypti (L.) larvae in laboratory bioassays in Veracruz, México. Four insecticide treatments and an untreated control were compared at low and high doses over 96 d of field exposure under full sun or full shade conditions: 1) microencapsulated Bti spore-toxin complex, 2) nonmicroencapsulated Bti spore-toxin complex, 3) a commercial Bti formulation, 4) a commercial formulation of temephos, and 5) an untreated control. The low and high doses corresponded to the LC₅₀ and LC₉₀ concentrations for the Bti insecticides and to 0.5 and 1.0 mg/liter for temephos; the corresponding values for the microencapsulated Bti and commercial Bti, estimated in this study, were 0.061 and 0.14 mg/ml and 0.13 and 0.30 mg/ml, respectively. Overall, the study demonstrated that microencapsulation with aluminum-carboxymethylcellulose improved the activity against Ae. aegypti larvae of B. t. israelensis spore-toxin complex over that of a nonmicroencapsulated spore-toxin complex and that the improvement was particularly important under full sun and high dose. Moreover, insecticidal activity of the microencapsulated B. thuringiensis israelensis spore-toxin complex was superior to that of a commercial B. thuringiensis israelensis formulation and comparable to that of the chemical insecticide temephos, Finally, it was suggested that the microencapsulated B. thuringiensis israelensis formulation should be evaluated for field use in Veracruz because its activity against Ae. aegypti larvae remained high through 31 d and this would allow halving of the current insecticide application frequency.