Abstract: Algae samples were taken regularly from the River Thames at Datchet from Oct. 1985 to Jun. 1986. All the 32 samples were identified. The results showed that there were 30 genera of algae belonging to 6 phyta in that area. Among them Bacillariophyta was most abundant, especially Stephanodiscus, which was the dominant species in the river. Some of the algae were quantitatively analyzed by counting the cells of algae, measuring the content of chlorophyll-a and determining the number and volume of total particulate suspended matter in the water. The results showed that, during the period for peak growth of algae, the total number of cells was as high as 15 558 cell/ml and the cell number of the dominant genus Stephanodiscus was 14 408 cell/ml, or 93% of tht total number. The average content of chlorophyll-a was 26.7 μg/L and the average number and volume of suspended particulare matter were 18 654 cell/ml and 8.37 μm3×106/ml respectively. From all the parameters above we can conclude that the river is already in the state of eutrophication.