The species composition and seasonal changes in the standing crop of planktonic rotifers were investigated at four stations in Lake Donghu, Wuhan in. 1991. The long-term species succession and abundance dynamics were described at two sampling stations, StationsⅠand Ⅱ. With the improvement of taxonomy system of Rotifera, some problems concerning the taxonomy of rotifers were clarified. The author observed 69 rotifer species in 1991, with 12 new records. In the water samples, 59 species were found. The maximum number of species was recorded at Station IV, and the minimum at Station Ⅲ. The number of species decreased from 66 in 1962 - 1963 to 38 in 1991 at Station Ⅰ, and from 54 to 38 at Station Ⅱ. There were 41 species that hatched from resting eggs. The number of species is maximum at Station Ⅱ, and minimum at Station Ⅳ. Species including Lepadella patella, Wolga spinifera, Mytilina compressa ect., which appeared in 1960's in Lake Donghu, vanished with the disappearance of macrophytes. The dominant species were different at different stations. Trichocerca pusilla has replaced Polyarthra trigra and Keratella cochlearis and become the first dominant species at Stations Ⅰ and Ⅱ. The standing crop of rotifers was high at Stations Ⅰ. and Ⅱ, lower at Station Ⅲ, and minimum at Station Ⅳ. The density of rotifers at Station Ⅰ has been in a state of equilibrium since 1980's, but it has been increasing rapidly at Station Ⅱ. Seasonal fluctuations in abundance showed a minimum in winter and spring, though the number percentage in these seasons was much higher in 1991 than that in the previous years. From the above-mentioned the results, the author is of the opinion that: 1. The degree of eutrophication is high at Station Ⅰ and Ⅱ; the environmental differences between these two stations were diminishing. The degree of eutrophication is lower at Station Ⅳ; the environment at Station Ⅳ is similar to that at Station Ⅰ 30 years ago. Station Ⅲ represents an intermediate type of environment. 2. Eutrophication and the succession of cladoceara are the major ecological factors affecting the changes in rotifers.