Abstract 1. 1. The acid production of gills from cold-adapted crucian carp ( Carassius vulgaris) is greater than in gills of warm-adapted fish. 2. 2. Organic-CO 2 production is not significantly influenced by temperature adaptation. 3. 3. A short photoperiod tends to increase organic acid production and decrease acid production in both warm-and cold-adapted animals. 4. 4. Glucose-6-phosphate dehydrogenase and catalase activities of gills were not altered by temperature adaptation. 5. 5. The percent inhibition of catalase by cyanide (10 −4) was unaltered by the temperature of adaptation. Catalase from gills of cold- and warm-adapted fish was completely inhibited by 10 −3 cyanide. 6. 6. The activities of 6-phosphogluconic dehydrogenase and aldolase were greater in gills from cold-adapted fish than from warm-adapted fish.