Phytoplankton cell density, biomass, chl-a concentration and their seasonal fluctuations at five stations in the channel between Dongting Lake and the Changjiang (Yangtze) River were studied from May 1995 to December 1997. The seasonal fluctuations had two peaks in a year in general. The annual patterns of total cell density and biomass indicated that diatoms contributed more than 70 percent of the total biomass (except from June to October) during the investigation period. Twenty-one dominant species contributed about 60-80 percent of the biomass in various months. The cell density, biomass and seasonal fluctuation were similar at Stations 1-4, but at Station 5 connected directly with the Changjiang River, the annual mean value's (127.34 x 10(4) cells/L and 0.46 mg/L in 1996, 41.98 x 10(4) cells/L and 0.2 mg/L in 1997 respectively) were lower than those (195.92 x 10(4) cells/L and 0.5 mg/L. in 1996, 132.96 x 10(4) cells/L and 0.57 mg/L in 1997 respectively) at Station 1-4; and the curve of the seasonal fluctuation sometimes did not coincide with those at Stations 1-4. Based on criteria for evaluating trophic status, the dominant species, annual average cell density values of 114-86 x 10(4) - 179.57x 10(4) cells/L, biomass of 0.48 - 0.5 mg/L, chl-a concentration of 0.42-0.51 mu g/L respectively recorded at five stations in 1996 and 1997, and values associated with the physico-chemical properties of the waterbody indicated that the water in the channel could be characterized as oligo-mesotrophic.