A comparative study of blood oxygen binding and carrying capacities of turbot Scophthalmus maximus and sea bass Dicentrarchus labrax , two fish species differing in their demand for oxygen, was carried out under three levels of chronic hypoxia ( P o 2 = 93, 65 and 40 mmHg) for 40 days. Blood O 2 affinity in normoxia was moderately high in both species ( P 50 was c . 12–13 mmHg at pH 7·7). The Bohr factor was significantly lower in turbot (−0·52) than in sea bass (−0·85). In both species, blood O 2 affinity was not significantly affected by oxygen depletion whatever its level and duration. In turbot, however, P 50 appeared to slightly decrease at the two more severe levels of hypoxia. In both species, blood O 2 carrying capacity was not affected by hypoxia and remained twice as high in sea bass than in turbot.