The sake yeast Saccharomyces cerevisiae Kyokai no. 7 (K-7) is capable of producing more than 20% (v/v) ethanol in culture. However, the activity of its alcohol dehydrogenase (ADH), which is a key enzyme in alcohol production, is less than that of the laboratory yeast S. cerevisiae X2180-1A. The ADH activities of brewery yeasts which were isolated and bred for the purpose of ethanol production were compared with those of laboratory yeasts. The ADH activities of a number of the brewery yeasts were found to be less than those determined for the laboratory yeasts. The level of ADH activity of each clone derived from a brewery yeast was also lower than that of each clone derived from laboratory yeasts when expressed in Escherichia coli cells. In the ADH1 gene of K-7, the region between the TATA box and ATG start codon and the region of polyadenylation in this nucleotide sequence were consistent with previous data. The ratio of intracellular ADH activity of K-7 to that of X2180-1A was about 1 : 2. On the other hand, the ratio of ADH activity of the clones derived from K-7 and X2180-1A, expressed under the control of the same promoter and in the same host, was about 1 : 1.3. The amino acid sequence of K-7 ADH1 was compared with previous data. The Glu(128) of K-7 ADH1 was unique among yeast ADH1s. Site-directed mutagenesis was carried out using ADH1 DNAs isolated from the sake yeast K-7 and the laboratory yeast X2180-1A, in order to determine amino acid alterations which affect activity. No single amino acid substitutions resulted in a change in the level of ADH activity. However, two double substitutions E128Q and E148Q or E128Q and V152I increased ADH activity to levels similar to that of X2180-1A Adh1. The substitutions of X2180-1A ADH1 to Q128E and Q148E or Q128E and I152V reduced ADH1 activity to the levels observed for K-7 Adh1.