The distribution of ostracod populations in Kagoshima Bay (South Kyushu, Japan) is analyzed with regard to environmental parameters. Topographic irregularity makes this elongated south-facing embayment amenable to subdivision into six different environments: Head, Margin, Slope, Basin, Mouth and Open Sea. The bay is under the influence of the Kuroshio Current, which enters from the east to flow northward along the west side of the Osumi Peninsula, and returns southward along the east side of the Satsuma Peninsula. A counterclockwise eddy current occurs in the widened central portion of the bay. The bay is in an area of andesitic volcanism centered in Mt. Sakurajima, an active volcano that rises from the water to separate the Bay Head area from the rest. The biocenotic indices such as abundance and diversity decrease from the Mouth of the bay towards the Basin and Head environments. On the other hand, volcanic ash and domestic input provide nutrients that are favorable to the ostracod biocenoses. The relative frequencies of species throughout the bay show that different environments are characterized by different assemblages. Correspondence analysis (CA) shows how ostracod distribution is influenced by the quality and structure of water masses such as dissolved oxygen content, salinity, anthropic pollution, and volcanically induced changes in pH.