The Oomycete fungus Crypticola clavulifera was pathogenic for laboratory-reared and field-collected larval Diptera, including the mosquitoes Aedes aegypti, Ae. notoscriptus, Anopheles farauti No. 1 and 3, Culex annulirostris, Cx. quinquefasciatus and the chironomid midge Chironomus tepperi. Zoospores were produced in water held at 15, 20 and 25 degrees C and were pathogenic for Ae. aegypti larvae, but at 30 and 35 degrees C zoospores were not produced.