This study was carried out to determine the effects of gradual salinity increase on osmoregulatory ability of the Caspian roach Rutilus caspicus, under conditions which mimic stocking conditions of hatchery‐raised fish. Initially, 30 juvenile fish (mean ±s.d. 3·20 ± 0·34 g) were transferred to 20 l circular tanks, in which salinities were changed in a stepwise fashion, from 0 to 5, 10 or 15 at 48 h intervals. The fish at salinity 15 were held for an additional 48 h at this salinity. Forty‐eight hours after salinity transfer, survival rate, haematocrit, plasma Cl⁻, Na⁺ and K⁺ concentrations, osmolality and gill Na⁺/K⁺‐ATPase (NKA) activity were measured. The only effect of exposure to 5 was a significant reduction in haematocrit compared to the freshwater control group. Exposure to salinity 10 raised haematocrit, Cl⁻ and Na⁺ concentrations and osmolality. At 48 h exposure to salinity 15, haematocrit, Cl⁻ and Na⁺ concentrations and osmolality were significantly higher than freshwater controls, and gill NKA activity was significantly lower, but the effect on NKA was no longer evident at 96 h exposure. There were no effects on survival. These results indicate that R. caspicus juveniles experience an initial non‐lethal iono‐osmotic perturbation following salinity increase but can adapt to brackish water at salinity 15.