Larvae of Aedes aegypti, Ae. vexans and Culiseta inornata were used to study the pathogenicity of the hyphomycete fungus Tolypocladium cylindrosporum. Experiments consisted of continuous exposure of second-instar larvae to 5 concentrations of conidia at 10, 15, 20 and 25 degrees C. The 3 test species were susceptible at all temperatures tested. There was generally a direct relationship between dosage and pathogenicity in the host population, however linear relationships between probit mortality at 10 days and log dosage occurred in only 11 of 37 assays performed. In these 11 assays, median lethal concentrations were in the order of 10(4)-10(5) conidia/ml. Median lethal times were between 3 and 14 days. It was concluded that the present bioassay method for T. cylindrosporum is inadequate and needs major improvement.