Abstract: The mid-lower Yangtze River basin has the most diverse molluscan fauna in China.However,owing to the increasing human activities during past decades,molluscs are highly imperiled in this region.Aiming at systematic assessment of status quo of mollusc species,investigations in 13 lakes of mid-lower Yangtze River basin were conducted during May,2003 to June,2005.69 species-were identified,of which 44 species were endemic to China.The diversity differed greatly between river-connected and-isolated lakes.Connected lakes were most speciose,especially potamophilus taxa,such as Rivularia spp,Semisulcospira spp and-Unionidae.In the two largest connected lakes,Lake Poyanghu and Lake Dongtinghu,the species numbers were 53 and 57,accounting for 76.8% and 82.6% of the total,respectively.In contrast,molluscs in isolated lakes were very species-poor,each with 4—19 species.Quantitative analysis indicated changes on relative abundance of two predominant species(Bellamya purificata and Corbicula fluminea) were consistent with connectivity of research lakes.By calculation of conservation value indices,threatened status of 67 species was evaluated.The results showed that the current status of molluscan fauna in Yangtze Lakes was grim.Of 67 species evaluated,58.2% were considered near threatened or threatened,of which 26 species were threatened,13 species were near threatened,and only 37.3%(25 species) were able to maintain stable populations.In comparison with historical records,it was found that the fauna had deteriorated significantly,and the dominant taxa of bivalves had changed from large-sized unionids to the small-sized clam,Corbicula fluminea.The deterioration is considered to be attributed to river-lake isolation,overharvest and water pollution.