The effects of haemoglobin-oxygen affinity and of cooperativity in oxygen binding on the oxygen extraction and ventilation requirement in fish are analysed using the mathematical model of Malte and Weber (1985). It is shown that a high affinity may allow a substantial reduction in ventilatory requirement and that optimal O2 uptake depends on a delicate balance between affinity and cooperativity. Using P50 and nH values from the literature, the oxygen extraction values predicted by the model for carp and rainbow trout coincide neatly with the values found for these species, indicating that the differences in oxygen extraction are a direct result of different blood oxygen binding properties.