A short DNA segment located in the kanamycin resistance region of plasmid Rl promotes site-specific recombination and plasmid maintenance. This segment has been reduced to 100 bp and subsequently to 44 bp without losing these properties. It can recombine with a similar segment located in the terminus region oftheEscherichia coli chromosome. It is proposed that this recombination is responsible for the plasmid maintenance properties of the Rl segment. The chromosomal site has been isolated; it also shows site-specific recombination activity. Sequence homologies were also found with a phage site-specific integration locus in the chromosome of Xanthomonas campestris and with the plasmid ColEl site-specific recombination locus. The recombinase required in all these systems is probably XerC, an E. coli enzyme acting on the cer site of plasmid ColEl for the conversion of plasmid dimers to monomers. It is postulated that site-specific recombination in the terminus region of the chromosome intervenes in the partitioning of the two daughter chromosomes.