Adaptive capacity of benthic microalga Attheya ussurensis (Bacillariophyta) from Peter the Great Bay, Japan Sea to the medium contamination with a salt of hexavalent chromium is tested in laboratory culture. In unpolluted culture, growth of the alga could be described by S-shaped curve, the lag-phase was short or absent, the exponential stage with square or near-square cells was characterized by high growth rate, mean size of cells was 16.0 x 15.5 mm at the exponential stage and 17.2 x 15.7 mm at the stationary stage, the chloroplasts were olive-green colored, had wide blades, and diverged radially from a cell center. After 10-days exposing to potassium dichromate in concentration 0.01 mg/L, the number of cells did not change but size and morphology of cells changed on the 7th day; the higher concentrations as 0.1, 1.0 and 2.0 mg/L caused the decreasing of cells number to 81, 48, 33 % of control number, respectively, and size and morphology of the cells changed on the 4th day for 0.1 mg/L and on the 2nd day for 1.0 and 2.0 mg/L. After further exposing under the same concentrations, only 27 % of cells survived under 0.01 mg/L, 10 % under 0.10 mg/L, and all cells were eliminated under higher pollution, cell division was inhibited in all cases, and the following morphological changes occurred: cell walls curved, cell horns shortened, chloroplasts deformed, cytoplasm consolidated, retraction was detected for 90 % of the cells. After the algae transfer from the medium polluted by potassium dichromate in concentration 0.01 and 0.10 mg/L to a clean medium, they only partially restored their number - to 42 % of control number for those exposed under the concentration 0.01 mg/L. The experiment shows that the benthic algae A. ussurensis is highly sensitive to the medium contamination because of breaking of the cells division process and morphological changes which cannot be restored even after cessation of the toxic impact.