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The life histories of two species of Gregarina, protozoans parasitizing Tenebrio molitor, were compared. Gregarina cuneata occurred primarily in larger larvae, whereas Gregarina steini was distributed evenly throughout all sizes of larvae. As larval size increased, an increase in prevalence of G. cuneata preceded a more substantial increase in parasite relative density. Sample variance/relative density ratios of the two parasite species also differed. When host larvae were isolated in a sterile bran medium, the prevalence of G. cuneata decreased, whereas that of G. steini remained relatively high. Trophozoites of G. cuneata were found predominantly in a paired condition, whereas trophozoites of G. steini typically were solitary.