Four industrial strains of bottom brewer's yeast and a group of their spontaneous respiration deficient (RD) mutants were tested for rates of metabolism of glucose and maltose under aerobic and anaerobic conditions. Qo2 of all the RD mutants tested (26 isolates) ranged from 1-9 to 6*8 jul/mg yeast dry weight on glucose and was lowered to about one-half on maltose although the original strains had the same Qo2 values on both sugars. No isolate showed any increase in glucose fermen tation in aerobic conditions as compared with the original strains and the decrease of Pasteur effect found in certain isolates was always accompanied by a strong decrease of glycolysis in both aerobic and anaerobic conditions. Two mutants showed a strongly negative Pasteur effect, for their fermentation rates were higher in aerobic conditions than in anaerobic ones. Two other mutants showed a strong negative Pasteur effect only on maltose. The ratio of Q*62 on glucose in most mutants was significantly lower than in their parental strains.