Adaptive abilities of the microscopic red alga Porphyridium purpureum (strain PP-AB11, isolated from the northwestern Japan Sea) are investigated experimentally by its repeated cultivation under various water salinity. Under the salinity of 8 ‰, the growth of P. purpureum was inhibited (0.1 division/day) during the first four days though there were no morphological differences of the cells as compared with those in the control (mean cell size was 6.3±1.2 µm in both cases); the cell density increased to 85 % of the control value after 21 days exposure. Under the salinity of 4 ‰, the lag-phase was also 4 days but size of the cells increased (on average up to 7.8 ± 1.5 µm), the cell morphology changed including the cytoplasm retraction, and contraction and granulation of the chloroplasts; the cell density increased less (to 53 % of the control value) after 21 days exposure. Under the salinity of 2 ‰, changes of the cell morphology and density were the most prominent: density of viable cells decreased to 6 % of the control value after 4-days exposure, size of the cells increased on average up to 8.7 ± 1.6 µm due to hydration, granulation was noticed in the chloroplasts, and the cytoplasm became dense. Under the salinity of 0 ‰, all the cells died after 4 days. Being cultivated repeatedly, the alga was better adapted to low salinity: after 21-days exposure the cell density was 97 % of the control value under salinity of 8 ‰ and 84 % of the control value under salinity of 4 ‰, with the growth rate 0.5 divisions/day and without visible morphological differences of the cells as compared with those in the control. It was adapted even to salinity of 2 ‰: the cells morphology was restored though the growth rate after 21-days exposition was still lower than in the control because of serious deterioration of the cells. The growth rate and cell morphology did not significantly differ from the control after reinoculation of the culture, previously adapted to salinity of 2 ‰, into the medium with salinity of 32 ‰.