Formulations prepared from 127 strains of Bacillus thuringiensis, including representatives from 16 variety/serotype groupings plus 2 unidentified strains, all of which were grown on a standardized medium, were assayed against larvae of Aedes aegypti, Ae. triseriatus, Culex tarsalis, Cx. pipiens and Cx. quinquefasciatus. Preparations from most strains were no more than moderately pathogenic to a host at high concentration levels (100 ug/ml. However, formulations from 26 strains showed marked effect against the most susceptible hosts. Ae. triseriatus, Cx. tarsalis and Ae. aegypti, with many found to have LC-50's well below 1 ug/ml. The most pathogenic formulations, labeled HD-169/R-567B and HD-96/R-574D, had LC-50's of 0.4 an 0.06 ug/ml, respectively, against Ae. triseriatus. Cx. pipiens and Cx. quinquefasciatus were much less susceptible, with only 23 formulations able to cause 50% mortality at concentrations between 4-10 ug/ml.