AbstractSilicified beyrichiocopid and podocopid ostracods from limestone nodules derived from the middle part of the Ichinotani Formation within the Hida Gaien Terrane of central Honshu Island, Japan, are associated with fusulinid foraminifera that indicate strata of the middle Moscovian (Pennsylvanian, Carboniferous). This is a rare record of ostracods from the Palaeozoic of Japan and the first systematic description of ostracods from the Carboniferous of the Hida Gaien Terrane. The fauna comprises six ostracod species (two new) assigned to the generaAmphissites,Kirkbya,Bairdia,AechminaandHealdia, and additional material of possible cavellinids. The numerical dominance of ornamented beyrichiocopids such asKirkbyaandAmphissites, along with smaller numbers of smooth podocopids such asBairdia, indicates an ‘Eifelian mega-assemblage’ ecotype (sensuG. Becker), that is typical of mid Palaeozoic shallow marine, high-energy environments in a fore-reef ecosystem.