The site-specific recombination mechanism through which the plasmid RP4 has been previously shown to integrate into the chromosome of Myxococcus xanthus has been investigated further. Once integrated in one of the numerous chromosomal sites from two different strains, through a precise site on the plasmid, the latter can be excised either precisely or after a definite 14.5-kb deletion. In some cases, the integration is followed by different DNA rearrangements that yield a higher rate of excision and integration. A model for the site-specific integration and excision of the plasmid is proposed.