In many countries molluscs are one of the main products of aquaculture. Microalgae are the most trophically valuable part of suspended organic matter in terms of filter feeders nutrition. The aim of this work is to investigate the nutritional spectrum of cultivated mussel Mytilus galloprovincialis at the Sevastopol sea-farm (Black Sea). To reach this goal the samples of sea water (1.5 litres) were thickened by reverse filtration to identify quantitative and qualitative compound of microalgae in native and concentrated sample. Molluscs were divided to different size groups. Their stomachs were prosected after sampling immediately and aliquot 0.02 µl was analyzed. Quantitative and qualitative compound of microalgae in water and stomachs was defined using light microscopy. This paper provides results of comparative analysis of quantitative and qualitative microalgae compound at the Sevastopol sea-farm water and stomachs of mussel Mytilus galloprovincialis cultivating at this farm. It is showed that species composition and amount of phytoplankton in water is typical for Crimean coastal waters. The similarity in microalgae compound between water samples and mussel stomachs reached maximum in Febriary (72%) and did not exceed 42% at the spring and autumn. Diatoms (27 species) and Dinophyta (11 species) occurred in the mussel food bolus frequently. Food spectrum of bivalves is quite wide but selectivity in filter feeders consumption is confirmed too.