Recombinant E. coli strains with 1-3 copies of lacZ genes on their chromosomes were constructed and their β-galactosidase (β-gal) expressions were examined. Serial dilution cultures were used to analyze the long-term genetic stability of the recombinant lacZ genes of the chromosomal or plasmid expression system. The strain with a 3-copy lacZ on the chromosome has a sustainable β-gal expression through 60 hours. However, the β-gal activity of the plasmid expression system lasted less than 36 hours under a no selection condition. bviously, the genetic stability of the chromosomal expression system demonstrated in this study is better than that of the plasmid expression system under nonselective condition, such as a medium without antibiotics. The results demonstrated that the strains with a multiple-copy-gene on the chromosome are useful for protein production in industrial repeated fed-batch fermentation.