Zoospore and oospore production of the entomopathogenic fungus, Lagenidium giganteum, was quantified from Culex tarsalis larvae in rice irrigation water. For the first through fourth instars, respectively, the average zoospore production (+/- SE) was 521 +/- 106, 2458 +/- 365, 10,546 +/- 582, and 14,360 +/- 1076, and the average oospore production was 17 +/- 1.8, 33 +/- 3.5, 44 +/- 2.6, and 34 +/- 3.1. Zoosporogenesis varied with the water sample. The ionic composition of the water samples was significantly correlated with zoospore production. The correlations, however, changed as different subsets of data were analyzed, suggesting that other factors also affected zoosporogenesis. The extent to which a cadaver was colonized by other microorganisms had a clear effect on the resulting Lagenidium reproduction. The interval of peak zoosporogenesis occurred between 24 and 36 hr postmortem for the first instar and 12 to 24 hr postmortem for other instars. Measurable production was observed up to 96 hr postmortem from fourth instar larvae.