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We examine the reproductive anatomy of both sexes of the isopod Thermosphaeroma thermophilum using SEM and light microscopy, and study the occurrence of multiple paternity using allozyme electrophoresis. Female reproductive openings are found ventrally, under the cuticular plate covering the body between the fifth and seventh pereonites. Receptivity for copulation is short during the sexual moult. Sperm from copulation are stored within the oviducts, close to the ovary, for up to two weeks before fertilisation and oviposition into ventral brood pouches. Sperm are not stored between broods. Male genitalia consist of penes with erectile extensions, and curved, channelled appendices masculinae. We suggest that appendices masculinae either channel sperm from the penes to oviduct openings located beneath the cuticular plate, or are used to push aside the cuticular plate for penile intromission. Paternity analyses suggest that multiple paternity is uncommon and that precopulatory guarding is an effective mate monopolisation strategy for males. We discuss the evolution of reproductive anatomy in the context of intersexual conflict, and suggest that conflict resolution may play a prominent role in the evolution of temporally restricted receptivity, male mate guarding, sperm storage, and delayed oviposition.