Four strains of bottom brewer's yeast used in Czech breweries did not differ substantially from one another in their frequencies of spontaneous respiratorydeficient (RD) mutants as tested by the triphenyl tetrazolium chloride (TTC) overlay technique. These frequencies amounted to 1.1-1.4% of the total viable cell number after cultivation in bottom layers of wort. The strain which had the highest rate of aerobic growth gave a spontaneous mutation frequency of 0.5% in shaken cultures.In comparison with the TTC overlay technique, YVL-Difco-agar with brom cresol green gave lower estimations of the frequency of production of spontaneous RD mutants.The RD mutants isolated can be divided into 11 groups on the basis of their cytochrome content (particularly cytochromes 6 and c) after aerobic growth.Of the 34 RD isolates tested, only 3 produced considerable numbers of revertants to respiratory sufficiency. These 3 mutants were derived from the same original strain, had the same cytochrome type, and had very slow growth rates.All RD mutants isolated had slower growth rates under aerobic conditions and reduced fermentation rates in static cultures in comparison with the parent types.