The ostracode fauna from the middle–upper Eocene Iwaya Formation on Awajishima Island, Hyogo Prefecture, southwestern Japan was studied to reveal a band of the Eocene ostracode faunal spectrum in the middle latitudes of the northwestern Pacific. Based on lithofacies and previously reported calcareous nannofossils, molluscs, and benthic foraminifers, the ostracodes were found to have inhabited inner shelf environments. The ostracodes entirely differ from the Eocene–earliest Oligocene faunas in Kyushu, southwestern Japan and the East China Sea, both in generic and specific composition. The difference is considered to be due to paleoclimatic and paleogeographic differences between Awajishima Island and more southern latitudes at that time.
Described species include three new species, Munseyella setouchiensis sp. nov., Hanaiborchella reticularitriangularis sp. nov., and Trachyleberis awajiensis sp. nov.