To evaluate the fish diversity of Yangtze River-connected lakes, a monthly survey of fish assemblage was carried out in theLake Dongting in the middle basin of the Yangtze River in March-December, 2004 and May, 2005. Three sampling areas along atransect from the Yangtze River mainstream to the lake center, i.e. from lake mouth (Chenglinji) to East Lake Dongting (Yueyang)and South Lake Dongting (Yuanjiang) were selected. Totally 69 fish species belonging to 6 orders, 14 families and 44 genera wererecorded. Cyprinids were the richest, accounting for 59.4% of total fish species. Species numbers and diversity indices showed thatfish diversity of the lake was high, with great spatial and temporal variations. Generally, higher diversities occurred in spring andsummer than in autumn and winter, and in South Lake Dongting than in the other areas. With regard to species numbers, the smallerthe distance between sampling area and Yangtze River mainstream was, the earlier the diversity peak was reached. Dominant specieswere determined as species with the value of dominance over 5000. There were 17 dominant species in the lake among which 80.0% were limnicolous species. With regard to ecological groups, limnicolous species dominated the lake, accounting for 74.0% of the totalspecies, while river-lake migratory species was the least, being 13.0% of the total. In comparison with that of 1970s, fish diversity ofLake Dongting had been dramatically decreased, especially for migratory species, and it was mainly due to the loss of habitats, decreasedfry resource and over-fishing. Analysis also showed that fish diversities in Yangtze River-connected lakes were much higher than thoseof disconnected lakes, indicating that river-lake isolation results in species decreasing. Thus, increasing river-lake connectivities isimportant to the conservation of fish diversity in the Yangtze River floodplain.