The ability of Culicinomyces conidia to persist and remain pathogenic to two mosquito species was studied in different aquatic environments in the laboratory. At 14oC conidia persisted and remained pathogenic to Aedes aegypti larvae for up to 112 days whereas at 25oC conidia were pathogenic for a maximum of 28 days. Mortalities of Culex quinquefasciatus larvae were lower than Aedes aegypti at 25oC. This was attributed to differences in feeding behavior. The short term effect of substrate type was minimal but larval mortality in cups containing coarse sand sediment fell at a faster rate than finer mineral sediments. This phenomenon may be due to conidia falling into the larger pore spaces and becoming inaccessible to mosquito larvae.