Ralstonia eutropha JMP134 (pJP4) is a useful model for the study of bacterial degradation of substituted aromatic pollutants. Several key degrading capabilities, encoded by tfd genes, are located in the 88 kb, self-transmissible, IncP-1 β plasmid pJP4. The complete sequence of the 87 688 nucleotides of pJP4, encoding 83 open reading frames (ORFs), is reported. Most of the coding sequence corresponds to a well-conserved IncP-1 β backbone and the previously reported tfd genes. In addition, we found hypothetical proteins putatively involved in the transport of aromatic compounds and short-chain fatty acid oxidation. ORFs related to mobile elements, including the Tn501-encoded mercury resistance determinants, an IS1071-based composite transposon and a cryptic class II transposon, are also present in pJP4. These mobile elements are inefficient in transposition and are located in two regions of pJP4 that are rich in remnants of lateral gene transfer events. pJP4 plasmid was able to capture chromosomal genes and form hybrid piasmids with the IncP-1 α plasmid RP4. These observations are integrated into a model for the evolution of pJP4, which reveals mechanisms of bacterial adaptation to degrade pollutants.