Abstract: Summarizing the data of phytoplankton surveys made in 1956-57, 1062-63 and 1973-75 in Lake Dong Hu, Wuhan, Hubei Province, the authors discuss the mainecological changes for the past two decades in phytoplankton association (abundanceand species composition, their annual and seasonal variations, diversity index andprimary production) at two sampling stations, which are representative of two differentregions, i.e., the pelagial zone and a bay region. Average annual abundance of plank-tonic algae was increased by several times, from hundreds to thousands of individualsper ml of water. Dominance of cryptomonads and diatoms was gradually replaced bygreen and blue-green algae, and there has been a marked increase of the larger formsof myxophycean species. In the pattern of seasonal fluctuation of phytoplanktonabundance the maximum population in early years was usually recorded in the springand in the autumn; from the 6th decade onwards, however, there has been a summerblue-green algae maximum, which becomes more prodominent and lasts longer than before.Markedly increasing is the annual and maximal diurna1 primary production, too. Allthese are indicative of eutrophication. On the basis of ecological changes in phytoplankton association as the main para-meter, the authors give a biological assessment of the eutrophication process in the Lake.Lake Dong Hu at its present stage can be regarded as an eutrophic lake on the way tobecoming a hyper-eutrophic one, and anthropogenic factors are mainly responsible forthe acceleration of the eutrophication process. From the angle of environmental protection and based on the characteristics ofshallow lakes, the authors make suggestion for the control of eutrophication in LakeDong Hu, which is of importance to water supply and recreation besides fishery.