In teleost fish, high speed ("burst") swimming is performed anaerobically while sustained performance is aerobic. The degree to which anaerobic energy production contributes to swimming performance in the steady exercise zone (swimming maintained for 2 to 200 min) in unknown. The relationship between oxygen consumption or scope for activity and swimming speed (U), the repayment of an oxygen debt during recovery from fatigue, and the reduction in maximum sustained speed (critical velocity, Ucrit) by reduced environmental oxygen or lowered blood haemoglobin suggest that steady performance in salmonids is aerobic. In salmonids, sufficient power is provided from the oxygen consumed to swim steadily at least at 80% of Ucrit. On the other hand, depending on the temperature, aerobic power output in cyprinids is only sufficient to reach 30–50% of Ucrit. Aerobic relationships holding for salmonids do not apply to cyprinids. In fact, high respiratory quotients early in exercise along with glycogen depletion and lactate accumulation rates in muscle during vigorous swimming suggest that steady performance in cyprinids is worthy of further examination.