Nucleotide sequences for the 18S rDNA of 28 cytherocopine ostracods that represent 16 families were determined and compared with those of bairdioidean and cytherelloidean ostracods. Resulting molecular phylogenetic trees consistently indicated that cytheroideans formed a monophyletic group and that bythocytheroideans are paraphyletic outside of the cytheroideans. This relationship suggests that the diagnostic morphological features of the Bythocytheroidea, such as the five adductor muscle scars and the first antenna with seven articulated podomeres, are plesiomorphic. The molecular phylogenetic relationships among cytheroideans indicated polyphyly of the amphidont basic type hingement, which is distributed in four lineages, i.e., the loxoconchids, leptocytherids, schizocytherids, and a group of hemicytherids, thaerocytherids, and trachyleberidids, suggesting that the hinge structure of the amphidont basic type has evolved at least four times independently in cytheroidean ostracods. The ostracod hinge structures may have evolved in concert with the extent of carapace calcification.