Cythere japonica was proposed by Hanai (1959) as a new species of the genus Cythere, from the Pleistocene Sawane Formation in Sado Island, Niigata Prefecture, Japan. The characters of carapace were already known, but the appendages have not been previously described. The authors formerly considered that this species should be separate from the genus Cythere sensu stricto because it has twice as many sieve‐type pore systems as typical Cythere species, and a markedly higher carapace. The existence of living Cythere japonica in the tidal zone of north‐west Japan is confirmed, and its taxonomic position re‐examined on the basis of its appendages and the ontogeny of pore systems. The appendages, except for the copulatory organ, are almost identical with those of other Cythere species, and their pore systems share the same pattern in and before the A‐4 moult stage. On the basis of these features this species should be retained in the genus Cythere. Phylogenetic relationships are considered on the basis of the ontogeny of pore systems. The abdominal segments of podocopid Ostracoda, which have always been regarded as difficult to observe because of their fusion, are shown clearly by the SEM. Copyright © 1991, Wiley Blackwell. All rights reserved