Oxygen uptake rate by rainbow trout red cells was measured in the temperature range 5° to 40° C. Between 5° and 20° C there was a steady increase in rate and at 20° C a maximum rate of 2.5 μl O2/min/ml red cells occurred, followed by a decline in the rate as the temperature increased. A morphological investigation of the red cell was conducted and possible functions of the nucleus in fish red blood cells are disc