Certain derivatives of copy-control mutants of plasmid Clo DF13 are not stably inherited in E. coli. These plasmids, predominantly present as multimeric DNA molecules, lack a specific region, designated parB. Here we present the nucleotide sequence of this parB region spanning 328 bp between 46% and 49% on the plasmid genome. parB is a noncoding region with extensive internal symmetry. A recA-independent, site-specific resolution process occurs between two intramolecular parB sites present in direct orientation relative to each other. A gene located in the direct vicinity of parB, gene L, is not essential for parB functioning. However, our genetic data indicate that transcription from the gene L-containing operon into parB is required. We conclude that the efficient maintenance of Clo DF13 cop derivatives containing parB is provided by resolution of mutimeric molecules. Because Clo DF13 wt and cop derivatives have a different response to the deletion of parB we postulate that two different recombination systems, a parB-dependent and a parB-independent system, operate in the efficient maintenance of Clo DF13 plasmids.