The life cycle of an intraerythrocytic hemogregarine, Hepatozoon clamatae, was studied in green frogs (Rana clamitans melanota), bullfrogs (Rana catesbeiana), northern leopard frogs (Rana pipiens), and in the mosquito, Culex territans. Gametogenesis, fertilization, and sporogony occurred within cells of the Malpighian tubules of laboratory-reared Cx. territans that had fed on naturally infected frogs. Mature oocysts containing hundreds of sporocysts were observed in mosquitoes 30 days postfeeding. Each sporocyst enclosed 4 sporozoites. Merozoites appeared in the peripheral circulation of laboratory-reared bullfrogs, green frogs and leopard frogs that had been fed sporocysts 35-70 days previously. Attempts to infect American toads (Bufo americanus) and blue-spotted salamanders (Ambystoma laterale) were not successful. Gamonts of this parasite induced nuclear fragmentation or segmentation in host erythrocytes. The life cycle, morphological, and morphometric features of H. clamatae are compared with H. catesbianae, a similar species that also infects ranids. Nucleotide sequence analysis of the internal transcribed spacer region (ITS-1) of these sympatric species revealed that only 6 nucleotide sites of the 129 base pairs of this region were variable among 4 isolates of H. clamatae and 2 isolates of H. catesbianae. A redescription of H. clamatae is presented based on data from this study and from the original description by Stebbins in 1905.