The plasmid pBR325 is a cloning vector constructed in vitro by addition of the chloramphenicol resistance (Cmr) gene of an IS1-flanked transposon to pBR322 (Bolivar, 1978). It is a 5995 bp plasmid carruing to sequence originating from ISl. DNA-sequence data suggest that its Cmr segment was derived from a Cm transposon longer than Tn9. The plasmid pBR325 carries between the Cmr and the Tcr genes a 482 bp sequence which duplicates, in the opposite orientation, a section of pBR322 located at the end of the Tcr gene. The same structure was found in pBR328, a deletion derivative of pBR325 (Soberon et al., 1980). The possible implications of this inverted duplication on cloning experiments are discussed.