This article focuses on the biochemical composition and the role of phytoplankton (primary producer) as a basic nutritional source in diets of fish. In general, an aquatic classical food web comprises phytoplankton, zooplankton and fish (planktivorous and predatory ones) as predominant elements, which in turn define the linear direction of an energy-flow pathway and nutritional transfer. Phytoplankton provides many valuable phytonutrients and biologically active ingredients, especially fatty acids, amino acids, sterols, organic minerals, enzymes, carotenoids, chlorophyll, trace elements, and vitamins, which are directly available for the first larval/juvenile stages of fish or indirectly (through trophic chains) for the more mature forms. Regarding the biochemical composition of phytoplankton, the most nutritional ones are lipids, which affect the growth, health and reproduction of aquatic animals, especially fish species. In general, the freshwater and marine chlorophytes and cryptophytes, and marine diatoms provide more polyunsaturated fatty acids (PUFA>SAFA>MUFA) than other groups, differently to fatty acids supplied by freshwater cyanobacteria (SAFA>MUFA>PUFA). Furthermore, cryptophytes have the highest n-3:n-6 ratio. A similarly high ratio is recorded in fatty acid composition of marine zooplankton with predominant PUFA. Addition of phytoplankton (e.g. Isochrysis galbana, Tetraselmis chui and Nannochloropsis oculata) to the process of fish larval rearing shows to improve the digestive functions at the pancreatic and intestinal levels. The most essential fatty acids for all fish are EPA and DHA, and the most popular fish species containing 4.08-11.84 mg g-1 of dry weight of EPA and DHA, as the sum, can be given in following order: Anguila anguila>Rutilus rutilus>Cyprinus carpio>Esox lucius>Blicca bjoerkna>Perca fluviatilis>Abramis brama>Sander lucioperca>Tinca tinca>Carassius carassius.