Abstract 1. 1. Cytochrome oxidase activity, determined over the range 10–40°C was higher at all assay temperatures in brain, gill and muscle homogenates of 10°C-acclimated goldfish than 30°C-acclimated fish, but was only higher in liver homogenates when assayed at 30 and 40°C. 2. 2. Thermal compensation of gill cytochrome oxidase did not appear to involve a significant increase in the mitochondrial content of the cells. 3. 3. Cytochrome oxidase, succinate-cytochrome- c reductase and NADH-cytochrome- c reductase activities were all higher in gill mitochondrial preparations from cold-acclimated fish. 4. 4. The concentration of cytochrome- c 1 was reduced in gill mitochondria of cold- compared to warm-acclimated goldfish. However, the concentrations of cytochromes- aa 3, - b and - c were similar in these two groups. 5. 5. It nis suggestes that mitochondrial electron transport systems compensate to the degree necessary to meet changing ATP demands of goldfish exposed to new environmental temperatures.