SUMMARY: Glucose-limited cultures of baker's yeast growing at 25° had a maximum growth rate, saturation constant and yield constant of 0·22 hr-1, 129 μg./ml. and 0·225, respectively, whereas when growing at 38° the corresponding values were 0·25 hr-1, 300 μg./ml. and 0·204. In continuous culture, with the dilution rate fixed at 0·1 hr-1 there were no differences observed in viability, incidence of respiratory deficient mutants, cytochrome spectra or mean cell dry weights, between cultures grown at 25 and 38°. The culture grown at 25° had a smaller mean cell volume, greater yield value and nitrogen utilization. Ethanol, pyruvate and α-ketoglutarate were secreted to a greater degree in cultures grown at 38°. Yeast grown at 25° had a smaller capacity to produce carbon dioxide but greater ability to take up oxygen. Enzymes associated with glycolysis, alcohol production, tricarboxylic acid cycle and respiratory chain in organisms cultured continuously at 25 and 38° showed few important differences. The most obvious ones were those involving α-ketoglutarate as a substrate, especially α-ketoglutarate dehydrogenase. There were only small differences in adenosine phosphates and nicotinamide nucleotides. At 25° the ratio NAD/NADH was 1·5 but for organisms grown at 38° the ratio was 1·1.