Sphingomonassp. CFO6 (a member of the alpha group ofProteobacteria) was isolated from a Washington soil by enrichment on the insecticide carbofuran as a sole source of carbon and energy. This strain has been shown to harbor five plasmids, at least some of which are required for catabolism of carbofuran. Rearrangements, deletions, and loss of individual plasmids resulting in the loss of the carbofuran-degrading phenotype were observed following treatment with heat or introduction of Tn5.Several putative insertion sequence elements of different sizes were cloned from these plasmids by trapping in pUCD800, a positive selection vector for isolation of transposable elements. Three of the most common putative IS elements (designated IS1412,IS1487,and IS1488) in the clone library were of different sizes and cross-hybridize with each other. An element hybridizing with IS1412,IS1487,and IS1488was mobilized during growth of CFO6 at 42°C and inserted into one of CFO6's plasmids (pCFO4), corresponding to a deletion in the plasmid and a loss of catabolic function. IS1412was completely sequenced and its sequence analyzed. IS1412is 1656 bp in length and possesses terminal partially matched inverted repeats of unequal length (17 and 18 bp). In addition, IS1412contains an open reading frame which encodes a putative transposase with significant homology to the putative transposases of IS1380fromAcetobacter pasteurianus,HRS1 fromBradyrhizobium japonicum,and IS1247fromXanthobacter autotrophicus.These related IS elements form part of a family of common IS elements distributed among members of the alpha group of theProteobacteria.