Isolates of Lagenidium giganteum from Northern Carolina (NC) and Louisiana (LA) optimally infected Culex quinquefasciatus larvae at 21 to 290C. Infection rates below 150C and above 320C were markedly reduced. Sporulatioin by the NC isolate in larvae was optimal from 19 to 320C, inhibited below 190C and completely absent at 100C. The LA isolate sporulated in larval cadavers less successfully; the optimum temperature was 180C. At temperatures above 210C the infected larvae were killed too rapidly for the fungus to attain maturation, and the Lagenidium perished. When cadavers with fungus of either isolate were subjected to 100C for days and then returned to 250C, the fungus successfully sporulated, but after 10 days at 100C, no sporulation occurred upon return to 250C.