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The degree to which parasites use hosts is fundamental to host–parasite coevolution studies, yet difficult to assess and interpret in an evolutionary manner. Previous assessments of parasitism in eugregarine–host systems suggest high degrees of host specificity to particular host stages and host species; however, rarely have the evolutionary constraints on host specificity been studied experimentally. A series of experimental infections were conducted to determine the extent of host stadium specificity (larval vs. adult stage) and host specificity among 6 tenebrionid host species and 5 eugregarine parasite species. Eugregarines from all host species infected both the larva and adult stages of the host, and each parasite taxa colonized several host species (Tribolium spp. and Palorus subdepressus). Parasite infection patterns were not congruent with host phylogeny, suggesting that host phylogeny is not a significant predictor of host–parasite interactions in this system. However, the two host stages produced significantly different numbers of parasite propagules, indicating that ecological factors may be important determinants of host specificity in this host–parasite system. While field infections reflect extant natural infection patterns of parasites, experimental infections can demonstrate potential host–parasite interactions, which aids in identifying factors that may be significant in shaping future host–parasite interactions.