Cell extracts ofVibrio marinus MP-1 grown at 15°C and suspended in either tris-HCl buffer or NaCl were compared for heat lability of hexokinase, glucose phosphate isomerase, and aldolase. In tris buffer glucose phosphate isomerase and aldolase lost 50 per cent activity after one hour at low thermal exposures of 34°C and 28°C, respectively. Complete loss of enzyme activity occurred at 36°C and 34°C, respectively. Hexokinase suspended in tris buffer lost all activity above 17°C. The cell extracts suspended in 3.6 per cent NaCl were more heat stable and the enzymes lost 50 per cent activity after exposure between 31°C and 35°C. The enzymes lost all activity on thermal exposure of 33°C to 38°C in the presence of NaCl.