We investigated how stenohalinic Black Sea Calanus euxinus, when migrating to the north-eastern Marmara Sea, can adapt to the pronounced salinity gradient there. During field observations (2005 to 2008) females of Calanus euxinus with significantly smaller body length (compared with those inhabiting the brackish Black Sea at 17 to 18 psu) were found in the deep, high-saline (similar to 38.5 psu) layers of the Marmara Sea. Similar diameters of eggs laid by small- and large-sized females and similar respiration rates in nauplii hatched from these eggs indicated the Black Sea origin of the C. euxinus population found in the Marmara Sea. To understand the morphological and physiological changes in C. euxinus during acclimation to high salinity, we studied the effect of salinity on behaviour, food consumption, growth patterns, hatching success and respiration rate of C. euxinus individuals collected in the Black and Marmara Seas. It was shown that the range of salinity tolerance for adult C. euxinus (17 to 30 psu) from the Black Sea was extended gradually to 40 psu after 1 d of acclimation to the high salinity of the Marmara Sea. Long-term parental salinity acclimation facilitates survival of eggs descending through the salinity gradient of the Marmara Sea, from brackish upper layers to deep, high-saline strata, or eggs laid in deep layers. While weight-specific respiration rates at the same temperature did not differ for C. euxinus juveniles from the Black and Marmara Seas, they were similar to 2 times higher in preadults and adults from the Marmara Sea than from the Black Sea. This may be the reason for the decrease in length and weight increments of these stages probably leading to the small-sized individuals of the seasonal C. euxinus population inhabiting the Marmara Sea.