Saccharomyces cerevisiae SOD2and CUP1genes were used to maintain high-copy number plasmids (YEp) in laboratory and industrial yeast strains. The plasmid, YEpS , containing the SOD2 gene was unstable in a sod2° mutant. However when Paraquat (0.5 mM) was used as a selective agent, the plasmid was maintained in the sod2° mutant but lost in the wild-type strain. When the CUP1 gene was inserted into YEpS 1 , the resulting plasmid (YEpCuS ) was 100% stable in the sod2° mutant grown in Cu -containing medium. In the absence of Cu , the proportion of plasmid-containing cells fell to 20%. YEpS was also transformed into an industrial strain, transformants could be selected in Paraquat-containing medium but showed poor stability.