The mechanisms of adaptation that allow the teleost Silurus glanis to maintain its resting oxygen consumption constant when the O 2 partial pressure (P O 2 ) m the inspired water (PI O 2 ) varied between 40 and 3kPa were studied at 13 °C. Steadystate values of oxygen consumption, ventilatory and circulatory flow rates, P O 2 in the inspired and expired water, P O 2 and O 2 concentration in the arterial and venous blood, haematocrit and acid--base status in the arterial blood were determined after 1-day exposures at selected PI O 2 values. Whole-blood O 2 -binding characteristics were also determined. The key adaptation after 1 day of acclimation was maintenance of oxygen consumption by ventilatory adjustment with no change in blood flow rate or pH (no Bohr effect). At each PI O 2 value (i) the ventilatory adjustment was minimal as the O 2 extraction coefficient from water always remained around 80–90 % and (ii) Pa O 2 stayed constant at about 2kPa. Data are compared with previous results in crayfish and other teleosts. It is concluded that the principle of a constant O 2 status in the milieu intérieur -- independent of large changes in PI O 2 for a given state of activity -- should be valid in many crustaceans and teleosts.