Gene mobilization events occur within and between cells in contaminated environments and are important for the adaptation of bacteria in degrading these contaminants. This has been demonstrated in a variety of ways using well characterized phenotypic markers and gene probes derived from bacteria that degrade hydrocarbons and chlorinated aromatic compounds, or that express resistances to toxic metal contaminants. Transposons and transposition genes may be useful genetic markers of adaptation. The insertion sequence IS1071 is found as a direct repeat flanking the chlorobenzoate catabolic genes of Tn5271 in Alcaligenes sp. strain BR60 from the Hyde Park site in Niagara Falls (Nakatsu, Ng, Singh, Straus and Wyndham (1991) P.N.A.S. (USA) 88, 8312-8316.). IS1071 belongs to the Tn3 family of transposable elements and is 3201 base pairs in length. An identical element has been reported to flank a chloroacelate dehalogenase gene on plasmid pUOl in Moraxella sp. strain B from Japan (Wyndham, Cashore, Nakatsu and Peel (1994) Biodegradation 5, 323-342; Kawasaki (1994) Chemistry and Biology (in Japanese) 32, 478-483). We have surveyed published restriction enzyme maps, the DNA sequence databases and our own collection of pollutant-degrading bacteria and have found that elements similar or identical to IS1071 are widely distributed in the Proteobacteria. A partial list includes (location and contaminant listed): Pseudomonas sp. strain LB400 and Alcaligenes sp. strain H850 (New York State; biphenyl/PCB); Pseudomonas putida strain PRS2015 (pAC27) (Philadelphia; 3-chlorobenzoate); Alcaligenes sp. strain CPE3 (pCPE3) (Italy; 3,4-dichlorobenzoate); Comamonas testosteroni strain T2 (Switzerland; p-toluenesulphonate); Comamonas sp. strain JS46 (Florida; 3-nitrobenzoate); Pseudomonas putida strain PP3 (United Kingdom; chloropropionate/dalapon); Pseudomonas putida strain mt-2 (Japan/Wales; pTDN1) (aromatic amines); Comamonas acidovorans strain UCC61 (pOPHl) Wales; o-phthallate); Pseudomonas pseudoalcaligenes strain POB310 (Germany; 4-carboxydiphenyl ether). A high percentage of bacteria from the vicinity of the Hyde Park chemical landfill site in the Niagara River watershed that degrade chlorobenzoates by three different pathways involving non-homologous genes (clc, cha and fcb) have been found to carry IS1071 in one or more copies. There is a high frequency of linkage between IS1071 and mer genes in a number of the Proteobacteria we have investigated. In many of the characterized strains, IS1071 elements flank catabolic genes on large, conjugative plasmids, suggesting IS1071 is frequently involved in the mobilization of catabolic genes or operons. In other cases IS1071 is found in single copy, on plasmids or in the chromosome of contaminant-degrading bacteria. In all cases the similarity is high, as indicated by high stringency hybridization, restriction enzyme mapping and/or sequence analysis. The frequency of IS1071 increases in bacteria from the sediment/water interface of microcosms treated with organic contaminants. IS1071 is therefore globally distributed and a useful marker of genetic adaptation in the Proteobacteria.