Microorganisms are of great importance to in water environment where they occur naturally and can be added artificially, fulfilling different roles. hey recycle nutrients, degrade organic matter, and, occasionally, they infect and kill the fish, their larvae, or the live feed. Also, some microorganisms may protect fish and larvae against disease. Hence, monitoring and manipulating the microbial communities in aquatic environments hold great potential; both in terms of assessing and improving water quality, but also in terms of controlling the development of microbial infections. To understand the aquatic microbial world the basic biological knowledge is essential. In this course, main definitions of aquatic microbiology are being given as well as microbial biodiversity