This monograph is the first floristic account of aquatic Oedogoniales of the northern part of Bihar. It covers descriptions of 101 taxa with 157 illustrations in the form of line drawings and 356 microphotographs on 89 plates. These taxa include 17 taxa under the genus Bulbochaete and 84 taxa under the genus Oedogonium. A total of 23 new species and 26 new varieties are described. In addition, 8 taxa are found as new records for Asia, 10 taxa are new records for India and 28 taxa are new records for the state of Bihar. A taxonomic key is presented for the identification of taxa. Distribution, phytosociology, and ecological observations of investigated taxa are also noted in the book.