This book is a taxonomic report on Plankton Algae found in samples collected from Southeast Asia covering Thailand, Malaysia and Cambodia. A total number of 670 taxa containing 66 taxa of Cyanophyceae, 26 taxa of Chrysophyceae, 27 taxa of Xanthophyceae, 8 taxa of Dinophyceae, 316 taxa of Euglenophyceae and 227 taxa of Chlorophyceae, has been described in the book along with photomicrograph and figures of each alga. In the algal group of Euglenophyceae, 10 taxa of Euglena, 28 taxa of Lepocinclis, 75 taxa of Phacus, 47 taxa of Strombomonas and 127 taxa of Trachelomonas have been found in polluted water, ponds or road side ditches of the area. The diversity of euglenaceous algae in the area has shown a relative connection with the distribution in Australia (Playfair, 1915, 1921), in Taiwan (Yamagishi, 1992), in China (Shi, 1999) and Japan (Yamagishi, 1984-1998). In other algal groups, Merismoarcus and Bacularia of Cyanophyceae, Phalansterium of Chrysophyceae, Octosporiella,Tetras poridium, Chlorophyceae have been observed as unique or rare algae.