The Escherichia coli K-12 chromosome contains six copies of insertion element IS1 at loci is1A-is1F. We determined their nucleotide (nt) sequences and found that they were classified into four types. Two copies of IS1 which flank a chromosomal segment containing the argF gene (IS1B and IS1C) have identical nt sequences. Another identical pair are IS1A and IS1E. Comparison of their nt sequences with the IS1 in plasmid R100 revealed seven nt mismatches for IS1A (or IS1E), two for IS1B (or IS1C), four for IS1D, and 75 for IS1F. The fact that the IS1s flanking the argF segment are identical supports the idea that the segment, together with the particular pair of IS1s, has constituted a composite transposon and transposed after genetic transfer from another bacterial species into E. coli K-12. Duplicated sequences were not observed in the regions flanking each of four copies of IS1, indicating that rearrangements have occurred in these chromosomal regions after IS1 elements had been inserted into several target sites. The four types of IS1 present in the E. coli K-12 chromosome were essentially similar to IS1s in plasmid R100 and in the chromosomes of Shigella strains. This and the above results suggest that they have been transferred horizontally from other Enterobacteriaceae, including Shigella, into E. coli K-12.