In China, the most diverse molluscan fauna occurs in the Yangtze basin. The molluscan fauna in this region are highly imperiled due to increasing human activities. Aiming at effective conservation of the deteriorating molluscan fauna, intensive field investigations of freshwater molluscs on the mid-lower Yangtze Lakes were conducted between June, 2003 and May, 2005. Altogether 69 species were identified during the collection, including 29 gastropod species belonging to 9 families, 40 bivalve species belonging to 5 families, and 42 endemic species. Among them, Unionidae and Viviparidae are the two families with the highest species numbers, accounting for 50.7% and 14.5% of the total, respectively. The diversity variesgreatly from lake to lake. River-connected lakes contain the most diverse fauna, especially those adapted to lotic habitat, such as Rivularia spp, Semisulcospira spp and some species of Unionidae. In the two large river-connected lakes, Poyang·Lake and Dongting·Lake, there are 66 species and 40 endemic species, accounting for 95.7% of the total species number and 95.2% of the total endemic species number, respectively. In contrast,the historical records, it is found that total species number and endemic species number decreased greatly, with a loss of about 50% of the historical records. Meanwhile, the distribution ranges of most taxa were also shrunk. Habitat loss and hydrological alteration in the Yangtze Potamo-lacustrine Complex Ecosystem caused by river-lake isolation should be the primary factor causing the decline of molluscan fauna. Other human activities, such as overharvesting, reclamation and pollution, may be also the important factors.