Aquatic fungi contribute to the energy flow and productivity of an aquatic and semi aquatic ecosystems by their active role in the utilization and biodeterioration of organic materials. These fungi also possess the ability to parasitize aquatic plants and animals including fishes under certain conditions. Physiology, environmental needs and activity of these fungi vary with their morphological characters. The taxonomy of these fungi can not remain static. The morphological variability of these species ultimately be linked to the genetic control of their characters. A considerable amount of taxonomic studies has been carried out by a number of mycologists all over the world. The present monograph shows occurrence, distribution, taxonomic characters and keys to the species of different genera belonging to Blastocladiales, Monoblepharidales, Saprolegniales, Leptomitales and Legenidiales isolated time to time from various ecological conditions of India. The volume would be suitable for post graduate students, teachers, researchers, administrative officers and scientists engaged in mycology, aquatic fungi, microbiology, plant pathology, fish biology, fisheries, water management and aquatic biology in India and abroad.