Species composition and abundance of Phytoplankton from 5 various classes of watershed in the Mae Klong River Basin were determined. The samples of represented subwatersheds covering the area of Kanjanaburi provinces were collected and analysed from October 1989 to March 1991. The results were as follows. The Phytoplankton in the Mae Klong River Basin were classified into 57 genera comprising 5 Phyla : 26 genera of Phylum Bacillariophyta, 16 genera of Phylum Chlorophyta, 12 genera of Phylum Cyanophyta, 2 genera of Phylum Euglenophyta and 1 genus of Phylum Pyrrophyta. However, the Phytoplankton were the same genera but varied in accordance with watershed classes. The fifth watershed class had the highest abundance of Phytoplankton in the Mae Klong River Basin, 6,482.67 cells/liter on average. The lower ones were those of the third, the first, the second and the fourth, for which the averages were 3,949.00, 3,833.33, 1,936.33 and 810.00 cells/liter, respectively. There were more Phytoplankton of the Phylum Bacillariophyta than of any other Phylum. According to the relationship between abundance of Phytoplankton and some water qualities, the abundance of Phytoplankton in some Phylum was related to some water qualities only especially in the Phylum Bacillariophyta. The results were that the stream water in the Mae Klong River Basin was in good condition with a high abundance of Phytoplankton which are the most important producers for Phytoplankton and will be of much benefit to fisheries. Therefore, we should have a suitable management program to protect these river basins to keep them in good condition and suitable for Phytoplankton abundance and useful for aquatic resources.