Ecological and physiological study of Calanus euxinus population from the Black Sea was carried out during the cruise of RN Bilim in Autumn 1996. The population was divided into two: active individuals, represented by all developmental stages, and diapausing CVs. The active CIV-CVIs underwent daily vertical migrations, while the diapausing CVs stayed at depth all day long. The population seemed to reproduce actively indicated by the large number of eggs and nauplii in the samples as well as the large proportion of spawning females (up to 80% of the total number of adult females). Egg production rate ranged from 5 to 25 eggs/female/day. The percentage of copepods with food changed with location and time; at night it varied from 10 to 97 in the upper layer, being less than 1 at the depth. Food remains was never observed in the guts of diapausing CVs. Up to 60% of gut in the upper layer, being less than 1 at the depth. Food remains was never observed in the guts of diapausing CVs. Up to 60% of gut contents consisted debris. The recognizable remains were parts of crustacean exoskeleton and dynoflagellates 10-60 mkm in size.Shipboard feeding experiments showed extremely low consumptions of chlorophyll (2.39-7.41 ng/ind/d in CVs and 5.67-14.33 ng/ind/d in females). The analysis of size classes distribution allowed the distinction of two groups among both diapausing and active CVs pointing on the heterogenity of the population. No difference between the length of males and females CVs was found. The state of gonad development differed between active and diapausing CVs. In the former gonad size ranged from 0.075 to 0.65 mm, in the latter small (0.1-0.15 mm) gonads before sex differentiation prevailed. An increase in gonad size was accompanied by a decrease in oil content. The active and diapausing CVs differed in their motor activity, respiration and excretion rates. Relationships between these indices and oxygen concentration were obtained. Under all oxygen concentrations studied, the indices for active CVs were higher than for diapausing CVs by a factor of 2 or 3.