The properties of the first translocatable element in Gram-positive bacteria, a 5.2 kb segment encoding erythromycin resistance in S. aureus, are described. This element translocates from plasmid to multiple chromosomal sites and from chromosome to multiple plasmid sites, sometimes causing insertional inactivation and deletion. The genetic control of translocation and its role in natural plasmid evolution are discussed and preliminary evidence for translocation of penicillin and chloramphenicol resistance is presented. In the latter case, translocation involves in intact plasmid.