An isogenic set of 11 recombination-deficient mutant strains of Bacillus subtilis has been constructed. Whereas plasmid pUB110 is stably maintained in such Rec- cells, the high copy number plasmid pC194 is unstable. Instability in Rec- strains could be mostly attributed to the deleterious effect of the presence of the plasmid on the Rec- cells' growth capability. In part, instability of pC194 derivatives could also be correlated with the presence of an unusually high amount of multimeric DNA molecules.