Factors which influence the stability of the recombinant trp^+ plasmid in E. coli were examined, and strategy for improving the stability of the recombinant trp^+ plasmid was investigated. For these purposes, many kinds of the recombinant trp^+ plasmid which differ in character of trp^+ gene such as attenuation deficiency and feedback inhibition resistance were constructed, and genes related to the tryptophan biosynthesis such as trpR, tyrR, tnaA, pheA and tyrA genes on E. coli chromosome were mutated. The stability of the recombinant trp^+ plasmid tends to decrease as the trp^+ gene expression increases. High concentration of tryptophan in the culture medium at the initial growth phase decreases the stability of the recombinant trp^+ plasmid. The stability of the recombinant trp^+ plasmid increased by the insertion of cer^+ gene into the recombinant plasmid.