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Four faunal associations of brackish and freshwater mollusks characterize the Early Cretaceous in Japan: Hauterivian-Early Barremian Tatsukawa and Shobu-faunal associations, Late Barremian Sebayashi-faunal Association and Early Aptian Hibihara-faunal Association. Differences between assemblages, at the specific level, are attributable to environmental variabilities: 1) Tatsukawa-faunal Association (Hauterivian-Early Barremian), yielding the Hayamina naumanni brackish-water and the Megasphaerioides okurodaniensis freshwater assemblages; 2) Shobu-faunal Association (Hauterivian-Early Barremian): yielding the Eomiodon nipponicus Assemblage; 3) Sebayashi-faunal Association(Late Barremian): yielding the Costocyrena radiatostriata Assemblage; and 4) Hibihara-faunal Association (Early Aptian): yielding the Costocyrena minor Assemblage.