Conidia and blastoconidia of the pathogenic hyphomycete Tolypocladium cylindrosporum were applied to temporary and semi-permanent pools in central Alberta on 6 occasions. Infections were detected in the field only following application of blastoconidia; indications were that infections did not occur in the field following application of conidia. Most infections were detected only after holding field-collected immatures under laboratory conditions for up to 24 days. Five species of mosquitoes were found infected. Infections occurred in collections up to 29 days post-application with most mosquitoes dying as 4th instars. No infections were detected during subsequent years. Infection rates of up to 33% for blastoconidia and 55% for conidia were estimated. It is speculated that field-collected mosquitoes succumbed to the fungus as a result of the stress of transfer to laboratory conditions.