The surfare-sediment diatom assemblages of 45 lakes in the middle and lower reaches of Yangtze River region were examined for the first time.The diatom abundance distribution pattern along the gradient of total phosphorus concentration and nutrition, as well as the result from DCA using the diatoms data, indicated that the diatom assemblages were responsed towards the changes of lake nutrition states.The assemblage dominated by multi-ecotypes mainly with epiphytic, facultative planktonic, planktonic and epilithic taxa was replaced rapidly by nutrient tolerant planktonic taxa when the lake comes close to be eutropicated.Consequently it is proposed that the occurrence of the assemblages combined with the taxa of the genera Cyclostephanos, Stephanodiscus and Cy-clotella meneghiniana, C.atomus could be an alert to the lake eutropication.Moreover,the diatom assemblages perfectly denoted the characters of the macrophyte-dominant lakes and the algal-dominant lakes, and were likely to be an effective tool forjudging the ecological type of lakes.