Abstract: Phytoplankton in the waters of the source regions of the Yangtze River and Lancang River was investigated in the summer of 2012. Altogether 29 species of phytoplankton were identified, consisting of diatom (17 species accounting for 54.83% of the total), green algae (7 species accounting for 24.14% of the total), blue-green algae (3 species), Cryptomonas (1 species) and Pyrroptata (1 species). The phytoplankton abundance at each sampling site was very low, only (9.14~13.60)×10⁴/L. There was no apparent difference of assemblages and abundance of phytoplankton between the source regions of the Yangtze River and the Lancang River. The results are close to the investigation results by Changjiang Water Resources Commission in 2010. The results show that the ecological environmental condition in the source regions is good in general and further researches in this aspect are of vital significance.

Key words: sources of three rivers, source of the Yangtze River, source of the Lancang River, phytoplankton, algae