1. The haematology of winter, summer, and autumn populations of carp has been investigated as a function of acclimation temperature.
2. Red-cell number, packed-cell volume, and haemoglobin tend to vary directly with temperature. Mean erythrocyte volume is increased at very low temperatures but, like mean erythrocytic haemoglobin and mean haemoglobin/unit erythrocytic volume, tends to be stable at moderate to high temperatures.
3. Although the data suggest a temperature-correlated increase in blood oxygen capacity, it is apparent that the animals must invoke other forms of cardiovascular-respiratory adjustment as well.
4. On a seasonal basis increases in oxygen capacity are seen at low, but not high, acclimation temperatures. It is suggested that this form of response may be related to the pattern of capacity acclimation typical of cyprinid fishes.