In these experiments were used the instrument Tox Y-PAM (Pulse amplitude modulation) (Walz, Germany) for study of contact between the algal virus (virus of Tetraselmis viridis - TvV), as viral suspension with infective titer 108 IE/ml, and Tetraselmis viridis algae cultures, isolated from marine environment of Black Sea and White Sea. Algae cultures were in logarithmic phase of growth and development. Contact of algae cultures with TvV showed a reduction in the efficiency of the primary processes of photosynthesis, Fv / Fm. The higher sensitivity of the photosynthetic device to viral oppressing action was marked in infected Tetraselmis viridis algae of White Sea in comparison with algae of Black Sea. Using of this device (instrument Tox Y-PAM) will allow to receive new knowledge about the ecology of algae and phytoplankton, about the role of a virus infection in such phenomena, as processes of photosynthesis and fluorescence, and also in interaction between the virus and host cells in an example of contact of algal virus and sensitive algae culture.