Major controls of the distribution pattern and abundance of living ostracod populations in Lake Illawarra, a coastal lagoon south of Wollongong, New South Wales, are salinity and the benthic flora. The biocenotic ostracod assemblage from the intertidal zone around Windang Island is a typical, diverse, upper sublittoral, open ocean fauna. The lake entrance channel, which is a transport corridor for marine sediments into the lagoon, has a restricted ostracod biocenose (14 species) but contains an additional 72 species in the diverse thanatocenose resulting from the mixing of estuarine and marine species. Within the lagoon, the benthic flora influences the ostracod distribution pattern with the most diverse assemblage (13 species) occurring in areas covered by seagrasses. Seagrass distribution is, in turn, controlled by water depth, circulation, turbidity and substrate. Estuarine ostracods associated with the seagrass beds can tolerate florally induced fluctuations in pH from 7 to 10 and in dissolved oxygen from 1 mg 1-1 to 14 mg l-1. In the deeper parts of the lagoon with a predominantly mud substrate, the ostracod assemblage is dominated by Osticythere reticulata. Most samples retrieved from the most polluted part of the lagoon contained no ostracods.A total of 90 ostracod species and subspecies belonging to 50 genera has been identified; nine species: Cytheralison cosmetica, Callistocythere janiceburrowsae, Callistocythere windangensis, Neocytherideis anneclarkeae, Actinocythereis robustus, Bradleya rectangulata, Procythereis jonesi, Hemicytherura windangensis and Cytheropteron wrightv, and one subspecies, Callistocythere dorsotuberculata paucicostata, are described as new to science. �� CSIRO 1987.