The E. coli tkt gene has been subcloned into high copy number vectors. In fed batch fermentations up to 4gL(-1) of soluble intracellular transketolase was produced representing 43% of the total cell protein. Increased plasmid stability during fed-batch fermentations was obtained by using kanamycin resistant pBGS vectors rather than the ampicillin resistant pUC vectors. Plasmid stability was maintained throughout growth in a complex medium without any selective pressure by incorporating the cer region from ColE1 into the expression construct.