Sixty−five ostracode species were obtained from 37 sufuace sediment samples collected from Lake Nakaumi between August 5 to 8，2002．The most dominant species is Bicornucythere bisanensis．This species is abundant in the central part of the lake，where oxygen-poor bottom waters develop in summer．Dolerocypria mukaishinense and Cytherura miii are sporadically dominant in relatively low-salinity environments such as river mouths and the Honjo-koku area．Other species are rare except at the lake mouth，where high-diversity assemblages composed of 47 shallow brackish to marine ostracode species are recognized．To clarify the relationships between ostracode assemblages and human activities over the last 40 years，particularly reclamation and freshening projects，we compared published ostracode studies of Lake Nakaumi，based on samples collected in 1963/1967 and 1986，with our present results．Six ostracode assemblages are distinguished on the basis of Q-mode cluster analysis．Ostracode assemblages and their distribution areas changed dramatically between 1963/1967 and 1986，related to change in pattern of bottom water circulation．In contrast，they have changed little at from 1986 to 2002，although the areas dominated by B．Bisanensis seem to be slightly enlarged．