At least one hundred and forty.six ostracode species were obtained from 29 surface sediment samples collected from Urauchi Bay, Kamikoshiki-jima, Kagoshima Prefecture, southwestern Japan. Four biofacies (IM, I, M, and O) were identified on the basis of Q-mode cluster analysis. Biofacies IM is distributed in the innermost part of the eastern inlet of the bay and contains many Xestoleberis hanaii. Biofacies I is distributed in the inner part of the bay and contains many Loxoconcha uranouchiensis. Biofacies M is found in the middle part of the bay and contains Loxoconcha tosaensis, Pistocythereis bradyi, Nipponocythere bicarinata, and Ambtonia obai, which are typical middle muddy bay species. Biofacies O is in the outer part of the bay and contains Loxoconcha japonica, Loxoconcha optima, and Neonesidea oligodentata, which live in areas under the influence of open water. Spinileberis quadriaculeata and Cytheromorpha acupunctata, which are typical inner muddy bay species, are restricted to part of the bay, whereas Bicornucythere species, which are typical enclosed inner-middle muddy bay species, are absent from the bay. These distributions suggest that there is little input of fine inorganic sediments or organic matter from the hinterland of Urauchi Bay because the bay is situated in small islands off the main islands of Japan, surrounded by hard sedimentary rocks, and no large rivers flow into the bay. Even when these sediments and organic matter enter the bay, they are transported to the deeper bay bottom or open seas due to the steep geomorphology along the coasts of the bay.