A pUB110-derived plasmid/Bacillus subtillis host combination was segregationally unstable when grown in chemostat culture with complex or minimal medium and under starch, glucose or magnesium limitation. The kinetics of plasmid loss were described in terms of the difference in growth rates between plasmid-containing and plasmid-free cells (dmu) and the rate at which plasmid-free cells were generated from plasmid-containing cells (R). Loss of plasmid-containing cells from the population was dmu dominated. Changes in medium composition and the nature of growth limitation caused variations in both dmu and R. The plasmid was most stable in glucose-limited chemostat cultures with minimal medium and least stable under starch limitation with complex medium. R and dmu were smaller for cultures in complex media than those in minimal media. Limitation by strach induced expression of the plasmid-encoded HT alpha amylase gene and was associated with increased values of R and dmu. Magnesium limitation in minimal medium caused a significant increase in dmu and a decrease in R.