With an increase in anthropogenic pressure on the ecosystem of the coastal zones of the sea, the relevance of assessing the state of plankton and benthos communities also increases. In the course of the study, the dynamics of the number of cells of the clone culture Thalassiosira excentrica Cleve 1903 (Bacillariophyta) has been determined under the influence of CuSO4·5H2O of various concentrations (32–1024 µg·L–1) under the conditions of 10-day experiments. In the control and at a low concentration of Cu2+, the proportion of living cells remains almost unchanged. The threshold concentration of copper ions for the survival of T. excentrica is 128 µg·L–1. At higher concentrations of Cu2+ (256–1024 µg·L–1), a sharp inhibition and death of the culture has been noted already within 1–3 days. The increase in the absolute number of cells at low concentrations of copper (32–64 µg·L–1) is described by the sigmoid response curve of the test-object. The growth values increase by 4–8 times on the 1st–5th day and decrease by 25–30 % by the 10th day. At Cu2+ concentrations of 128–256 µg·L–1, there is no positive increase in the number of cells on days 1–5; on subsequent days, it decreases by 2 times or drops to zero. The intensity of the specific cell growth significantly decreases depending on the increase in the concentration of the toxicant. It has been found that on days 1–5, the specific increase is positive in the range of copper concentrations of 32–128 µg·L–1 and negative – at concentrations of 256 µg·L–1 and higher. In the period of 5–10 days, for all concentrations of the toxicant, the values of the specific increase are negative. The obtained results make it possible to recommend T. excentrica as a promising test object for toxicological experiments.