We present the sequence of a 3500-bp region of the Escherichia coli strain K12 chromosome lying between the tryptophan operon and the tonB gene. Analysis of the sequence yields six open reading frames that have properties characteristic of genes for proteins. The reading frames are closely spaced, and putative transcription units and control sites compose over 95% of the DNA. The sequences of several wild strains of E. coli have been determined for a large segment of the region described. Comparison of these sequences reveals the effects of base substitutions, DNA rearrangements, and recombination. In the regions presumably expressed as polypeptides, most of the natural variation results from synonymous substitutions. However, the DNA rearrangements identified have end points within the open reading frames and disrupt them in a variety of ways. The effects of genetic recombination between strains, recently found to be significant on a large scale in E. coli, are also apparent in the region between trp and tonB.