A Helicosporidium near parasiticum isolated from field-collected larvae of Culex nigripalpus infected larvae of 14 mosquito species in 6 genera and also 6 species of Diptera, Coleoptera, and Lepidoptera. The most susceptible mosquito hosts were Anopheles albimanus, An. quadrimaculatus, and Cx. salinarius. Because of differences in host specificity and pathology, and in spore morphology, the Helicosporidium of mosquitoes is apparently separate from both the Helicosporidium reported from a beetle and H. parasiticum as originally described.