Saccharomyces cerevisiae SOD2 and CUP1 genes were used to maintain high-copy number plasmids (YEp) in laboratory and industrial yeast strains. The plasmid, YEpS(1), containing the SOD2 gene was unstable in a sod2 degrees mutant. However when Paraquat (0.5 mM) was used as a selective agent, the plasmid was maintained in the sod2 degrees mutant the CUP1 gene was inserted into YEpS(1) the resulting plasmid (YEpCuS(1)) was but lost in the wild-type strain. When 100% stable in the sod2 degrees mutant grown in Cu2+-containing medium. In the absence of Cu2+, the proportion of plasmid-containing cells fell to 20%. YEpS(1) was also transformed into an industrial strain, transformants could be selected in Paraquat-containing medium but showed poor stability.