1. 1. Isozyme induction was demonstrated for the enzymes glucose-6-phosphate dehydrogenase and lactic dehydrogenase in the liver of the creek chub, Semotilus atromaculatus, acclimated to varying conditions of temperature and photoperiod. Other tiflaes examined, including gill, muscle, brain and heart, reflected tiflae-specific patterns, but no instances of isozyme induction. 2. 2. Electrophoretic analysis of liver G-6-PDH indicated that, regardless of photoperiod, warm acclimation (20°C) induced two isozymes which were only negligibly present under conditions of cold acclimation (5°C). All other tiflaes examined displayed only a single band of activity which, based on relative mobility and staining intensity, appeared similar to G-6-PDH-1 found in liver. 3. 3. A total of 11 bands were noted for liver LDH under most conditions of acclimation. A comparison of liver LDH banding patterns between 5°C 8L–16D and 5°C 0L–24D indicated a complete reversal in staining dominance between LDH isozymes 10 and 11; while acclimation to 5°C 16L-8D yielded a pattern in which LDH 10 was completely absent. 4. 4. These results suggest that isozyme induction as a molecular mechanism of thermal compensation, may be limited to specific enzymes in specific tiflaes, such as liver. Undoubtedly, photo-period also plays some role in thermal compensation at the molecular level.