Mosquito fungal pathogens, Metarhizium anisopliae and Tolypocladium cylindrosporum, were compared with regard to virulence against the larvae of Aedes aegypti, Anopheles stephensi and Culex pipiens. Culex pipiens larvae were much more susceptible to M. anisopliae conidia than An. stephensi or Ae. aegypti. But Ae. aegypti and Cx. pipiens larvae were equally susceptible to T. cylindrosporum propagules which weakly attack An. stephensi. Using a high concentration conidial suspension (10(7) sp/ml) of M. anisopliae no. 139, Ae. aegypti larvae were killed immediately within 1.1 days, before intrahemocoelian invasion; but at lower concentrations (10(6) and 10(5) sp/ml), typical mycosis occurred. However, T. cylindrosporum no. 3 blastospores were much more pathogenic to Ae. aegypti larvae than conidia. Conidial suspension of 10(7) spores/ml killed 68% fourth-instar larvae, relative to the 96% invaded by blastospores under the same conditions. Presoaked conidia virulence appeared still intermediate between conidia and blastospores. At low temperatures, 15 degrees C, virulence of M. anisopliae highly decreased, while at the same temperature, T. cylindrosporum blastospores were still virulent.