The freshwater unionoid mussel Popenaias popeii is restricted to river drainages of the Gulf of Mexico in southwestern North America. Previously, this species has been classified as both a short-term and long-term larval brooder based on limited data. Histological analysis of gonadal tissue from specimens collected in New Mexico indicated gonochoric individuals; hermaphroditism was not observed in any specimens. Although male and female gametes were present for almost the entire year, gonadal activity appeared lowest in October and November, when tissue recovery and reorganization were more evident than gametogenesis. Gravid females were observed from March through August, with all stages of embryos and larvae present on every sampling date during this period. Non-gravid females always were present, even if they contained active gonads. Contrary to previous reports, P. popeii is considered an asynchronous short-term brooder with an extended period (late winter to mid-summer) of oviposition. A small percentage of gravid females used posterior portions of inner demibranchs as marsupia. This character suggests that inner demibranchs might function in a facultative capacity as an overflow reservoir for larvae unable to fit into full marsupia of the specialized outer demibranchs.