pWW53 is a 110 kbp catabolic plasmid which encodes the complete pathway for the utilization of toluene and the xylenes. The upper pathway operon xylCAB is located between two homologous but distinct meta pathway operons, xylDLEGF(I,J,K)H, which are in direct repeat. These have each been cloned on large HindIII restriction fragments HA (17.5 kbp) and HB (15.6 kbp), the restriction sites of which have been mapped. During growth of MT53 on benzoate, mutants which have lost the ability to grow on hydrocarbons such as m-xylene (Mxy-) but which retain the ability to grow on their carboxylic acid metabolites such as m-toluate (Mtol+) take over the culture before ultimately being displaced by plasmid-free strains which are Mxy- Mtol-. The plasmids in the Mxy- Mtol+ mutants are formed by a large deletion between homologous regions of the two duplicate meta pathway operons. This causes the loss of the intervening xylCAB operon and the formation of a hybrid xylDLEGF(I, J, K)H operon, starting with the genes originally on HA and terminating with the genes originally on HB.