Bacillus thuringiensis serovar medellin strain 163-131 and Bacillus thuringiensis serovar jegathesan (B.t.jeg.) strain 367 are very toxic to mosquito larvae. However, they are 10 times less toxic than Bacillus thuringiensis var. israelensis (B.t.i.) to mosquito larvae under laboratory conditions. Lyophilized powders were produced from these strains and their toxicities were compared to that of powder produced from the B.t.i. strain. Larvicidal activity was titrated using Aedes aegypti (Bora-Bora strain) larvae, with IPS82 powder as the standard. The efficacy of these powders in the field was determined using Culex pipiens (Montpellier strain) in Paris, France, and Ae. aegypti larvae (French Guiana strain) in Cayenne, French Guiana, in standardized conditions. Residual activity was also assessed in the laboratory, using Cx. pipiens (SLAB strain), in Montpellier, France. Any negative effect of direct sunlight, soil, or polluted water on the residual activity of the 3 powders was recorded. Increasing bacterial concentration by a factor of 8 had little effect on the duration of larvicidal activity, except in the presence of polluted water and when substrate was added. All powders had similar initial efficacies against both types of mosquito larvae, in all conditions except water rich in organic matter. Bacillus thuringiensis serovar medellin had the lowest residual activity, both in the laboratory and in the field, whereas B.t.jeg. remained toxic for as long as B.t.i.