Laboratory infections of Aedes taeniorhynchus and Culiseta inornata larvae have been obtained with two variants of Coelomomyces psorophorae. No infections resulted when larvae were incubated 24 hr with sporangia and zoospores. When four consecutive groups of larvae were incubated for 6 days each in containers to which dehiscing sporangia, sod, and algal water were added with the first group, no infections occurred in the first group of larvae, but infections did occur in the second group. This delay in infective activity suggests that alternate stages in the life cycle of these fungi may exist which lead to the development of unknown undescribed infective units. When A. taeniorhynchus larvae were incubated with sod collected 2 wk previously from a field site which consistently produced C. psorophorae infected larvae of A. taeniorhynchus, infection was obtained within the first 24 hr of exposure.