An industrial polyploid strain of Saccharomyces cerevisiae containing Saprolegnia ferax beta-amylase gene was developed by using two yeast integrating plasmids. One plasmid was constructed that contains the geneticin resistance gene (Gt(r)) as the selection marker and the ribosomal DNA (rDNA) portion that comprises the 18S rDNA as the recombination site. The other plasmid contains the aureobasidin A resistance gene (AUR1-C) as the selection marker and the chromosomal Ty delta sequence as the recombination site. The beta-amylase activity of one clone of Saccharomyces cerevisiae transformed sequentially with these two plasmids was approx. 9 times higher than that of Saprolegnia ferax. This type of integration was mitotically stable even after 100 generations of cell multiplication under non-selective conditions.