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The glacial relict Saduria (Mesidotea) entornon (L.) (Crustacea: Isopoda) lives buried in sandy/muddy bottoms in the Baltic. During hypoxia Saduria remains buried until oxygen tension (P,Oz) has decreased to < 5 Torr (8°C; 7 % S). Respiration rate (MO2) for buried Saduria was lower than for other crustaceans of similar size. Saduria is able to maintain a stable MO2 with decreasing P,Oz down to < 5 to 10 Torr, i.e. in practice over the entire P , 0 2 range. After exposure to severe hypoxia for many hours a respiratory overshoot was sometimes found. Only small amounts of haemolymph lactate had accumulated at P,02 = < 5 Torr; up to 30mg 100ml-' after 144 h exposure. The presence of anaerobic endproducts other than lactate is suggested. Patterns of heartbeat frequency (fh) and gill ventilation (6) changed considerably, but gradually, with time in buried Saduria. Normoxic fh varied from 0 to 120 beats min-' in a cyclic sequence lasting 5 to 6 min. f, varied in a similar way although with different rates. At P,Oz = < 10 Torr the acute ventilatory response was a high stroke frequency, but after some hours the ventilatory pattern changed to the same cyclic sequence as at higher P,,02, f, retained the same cyclic sequence in hypoxia as in normoxia but without cardiac arrests. Cardiac output remained constant over the entire P, O: ! range but ventilatory efficiency increased with decreasing P,02. Respiratory independence is governed by changes in ventilatory pump flow and in behaviour in order to facilitate the transfer of oxygen from the often hypoxic Baltic bottom water to respiratory tissues.