A standardized air dried spore and crystal preparation of Bacillus thuringiensis var. israelensis (IPS-78) produced at the Pasteur Institute, Paris was bioassayed under laboratory conditions against late instars of Culex quinquefasciatus and five other mosquitoes found in the vicinity of Manaus, Brazil. The LC50 and LC95 for Cx. quinquefasciatus were .042 and 0.33 ppm, respectively. When an LC100 concentration was administered to Cx. quinquefasciatus larvae, mortality was noticeable after 2 hrs and was complete within 12 hrs. The primary powder, R-153.78 produced 98.3% mortality at 0.1 ppm compared to 65.0% for the standard at the same conc. Five peridomestic and sylvatic mosquitoes responded variably to 0.1 ppm of the standard. No mortality was produced in Cx. (Carrollia) sp.; Trichoprosopon digitatum responded with 43.3% mortality and Cx. mollis and a mixture of Limatus durhami and L. flavisetosus responded with 63.3% and 63.6% mortality, respectively.