Oxygen consumption, anaerobic metabolism, and oxygen supply of inner tissues were analysed in Sipunculus nudud at different oxygen tensions. Oxygen consumption, energy expenditure, and the PO2 in the coelomic fluid decreased linearly with declining ambient PO2. Below a certain range of PO2, which was a function of the size of the animals, the rate of oxygen consumption deviated progressively from the linear PO2/Mo2 function. In the same range of ambient PO2 the coelomic PO2 levelled off. Anaerobic glycolysis, phosphagen degradation, and the succinate-propionate pathway became apparent with concentration changes of anaerobic metabolites first during in inner tissues. In extension of the conventional definition (Prosser, 1973; Dejours, 1981) the term critical PO2 (Pc) is applied to the oxyconforming Sipunculus nudus. The Pc is redefined as the steady-state PO2 below which environmental oxygen availability becomes insufficient for complete aerobic metabolism (as indicated by the onset of anaerobic energy production). It is discussed to be closely linked to the oxygen supply of inner tissues. This redefined critical PO2 is shifted to higher partial pressures with increasing size of the animals because of the diffusion distance related decrease in coelomic PO2. Accordingly, with the decline of ambient PO2, oxygen starts to be released from haemerythrin at higher ambient PO2 values in larger animals. The pigment, which is likely to function as an oxygen store, defers anaerobiosis and, thereby, supports compensation of a higher Pc in large individuals by means of an increased haematocrit. The Pc is discussed as crucial factor for survival of individual animals in intertidal oxygen-depleted environments. © 1985.