The present invention describes a mutant plasmid replication control region having the ability to convey a phenotype of altered plasmid copy number to the plasmid on which it resides. The mutant replication control region is based on a similar region isolated from the pBBR1 plasmid family. Plasmids containing this replication control region cannot be classed as belonging to any known incompatibility group and thus may co-exist with a broad range of other plasmids in a single host.