1. The technique of microacrylamide slab gel electrophoresis was used to repeatedly monitor the qualitative isozyme composition in the tube feet of the sea urchin, Arbacia punctulata exposed to different temperature regimes. 2. Four enzyme systems were assayed. Three (MDH, HK, and ACPH) were monomorphic for each animal studied and no change in band migration pattern was observed. Banding patterns for the fourth system (EST) were observed to vary in the same individual. 3. The reversible induction of esterase variants in the tube feet of the same individual is reported. 4. The change was temperature dependent, and a period of 7-14 days acclimation was required to induce the pattern alteration.