The taxonomic composition, abundance and biomass of microzooplankton were studied at eight stations in Lake Shinji-Ohashi River-Lake Nakaumi brackish-water system, Japan, monthly from April 1998 to March 1999. Over the entire area, naked ciliates numerically dominated the microzooplankton community (annual mean 39. 6%) followed, in order, by tintinnids (30.3%), copepod nauplii (24.6%) and rotifers (5.5%). The abundance of each taxonomic group of microzooplankton varied geographically due to large salinity variations (range 1.5-33.3 p.s.u.). It was notable that naked ciliates occurred overwhelmingly in Lake Shinji (54.9% of total microzooplankton) and rotifers were relatively numerous in Lake Shinji (8.8%) and Ohashi River (11.1%), where the salinity was lower (annual mean 4.1 and 13. 6 p.s. u., respectively) than in Lake Nakaumi and Sakai Strait (26.3 and 29.8 p.s.u., respectively). Owing to large seasonal temperature variation (range 5.4-29.8°C), the abundance of microzooplankton showed marked seasonal variations, being higher in spring and summer than in the remaining seasons. A total of 49 species of tintinnids were identified, and 15 of these species reached concentrations >500 individuals l -1 . The occurrence of most tintinnid species was confined to certain months or locations, closely associated with species-specific temperature and salinity preference and/or tolerance. In this eutrophic system, food supply for microzooplankton might be sufficient due to extremely high chlorophyll a concentration (annual mean 8.8 μg l -1 in Sakai Strait to 22.6 μg l -1 in Lake Shinji). However, microzooplankton biomass remained moderate (range 0.19-18. 7 μg C l -1 ) due probably to heavy predation by mesozooplankton, which inhabit this brackish-water system at extremely high biomass.