Rates of oxygen consumption (M(O(2))) for Fundulus grandis, the gulf killifish, were measured in air-saturated water, at four progressively lower levels of oxygen and upon normoxic recovery. The pattern of M(O(2)) versus oxygen partial pressure (P(w)O(2)) was that of an oxygen regulator, with a critical oxygen pressure (P(c)) of 34 torr (1 torr=133.3 Pa). Below this value, M(O(2)) decreased and the concentration of blood lactate increased, indicating anaerobic metabolism during hypoxia. Recovery was characterized by elevated M(O(2)) compared to the initial normoxic exposure, coupled with the rapid clearance of blood lactate. Variation in M(O(2)) among the individual fish was appreciable and, in general, it was greater at higher levels of P(w)O(2). This inter-individual variation was significantly larger than the variation between replicate measures of M(O(2)) for a given individual, i.e. it cannot be attributed solely to random error. Furthermore, values for M(O(2)) during normoxia were found to be repeatable when the same fish were used in multiple experimental trials. The observation of significant, repeatable inter-individual variation in M(O(2)) suggests that such variation is a real and potentially important feature of fish metabolism.