We have studied the influence of sigma s on the stability and number of copies of the promiscuous plasmid pLS1 in Escherichia coli. Our results indicate that pLS1 is less stable and has a lower number of copies in a rpoS mutant than in a wild-type strain during stationary phase. This behaviour does not seem to be due to differences in the expression of pLS1 replication regulators, but to be related to plasmid topology.