Mutant P1 Ap Cm lysogens were isolated in which the drug resistance genes resident on the plasmid prophage P1 Ap Cm are amplified by a novel mechanism. The first step required for amplification is IS1-mediated rearrangement of the P1 Ap Cm prophage. The drug resistance genes are amplified from the rearranged P1 Ap Cm prophage by the formation of a plasmid (P1dR) which contains the two resistance genes. The P1dR plasmid is an independent replicon about one-half the size of P1 Ap Cm that can be maintained at a copy number eightfold higher than that at which P1 Ap Cm can be maintained. It contains no previously identified replication origin and is dependent on the Rec+ function of the host.