The effects of exposure to sublethal concentrations of Bacillus thuringiensis var. israelensis (Serotype H-14) on second instar Aedes aegypti larvae were investigated. A test system was developed in which adverse effects would be detected as increased duration of larval development and decreased adult body size. No evidence of negative effects on survivors could be detected when sufficient B.t.i. dosages were applied to kill approximately half of the larvae in the treatment groups. However, when larval density was not controlled, and competition for food decreased as a result of larval mortality in the B.t.i.-treated groups, adult wing length was greater in the B.t.i. survivors than in the untreated controls. In addition, a residual mortality was noted in larvae that had been exposed to B.t.i. for 24 hr and then removed to habitats without B.t.i.