A PUB 110-derived plasmid encoding chloramphenicol resistance, kanamycin resistance and high-temperature α-amylase showed a high degree of segregational instability when inserted into Bacillus subtilis. In an attempt to obtain stable derivatives, the organism was grown in chemostat culture in the presence of chloramphenicol. It was periodically found necessary to increase the concentration of chloramphenicol in the medium feed in order to avoid plasmid loss. Strains were isolated after 19 and 160 generations, which showed high levels of plasmid stability. This characteristic appeared to be genotypic. No detectable difference in plasmid copy number was found between the original and the improved strains. The stability characteristics resided in the host, rather than in the plasmid. Stable isolates possessed elevated MICs for both chloramphenicol and kanamycin. Their maximum specific growth rates were higher than that of the original strain, and similar to that of the plasmid-free parent strain.