Diploid strains homozygous for both MAT allele and STA genes (a/a, STA1/STA1 or STA2/STA2 or STA3/STA3) and diploid strains homozygous for MAT allele but intercombinative for STA genes (a/a, STA1/STA2 or STA2/STA3 or STA1/STA3) were constructed by means of the protoplast fusion or the colchicine treatment. According to glucoamylase activity in YPS medium, we studied the gene-dosage effect and their interrelation of these three polymeric genes coding for glucoamylase. The results of the glucoamylase activity determination showed that the gene-dosage effect of glucoamylase is obvious in diploid and triploid strains homozygous for both MAT allele and STA gene, such as the glucoamylase activity of a diploid strain SFY56-6 and a triploid strain SFY56-104 homozygous for both MAT allele and STA genes were respectively 2.35 and 3.18-fold as compared with that of the their parental strain IATA-Y56 that is a haploid Saccharomyces diastaticus. Moreover, the glucoamylase activity of diploid strains homozygous for MAT allele but intercombinative for STA genes also showed the combinative gene-dosage effect to a certain extent.