In the Escherichia coli K-12 chromosome, six copies of IS1 (IS1A - IS1F) have been identified and characterized. According to their nucleotide (nt) sequences, the six IS1 copies can be classified into four types, IS1A(IS1E), IS1B(IS1C), IS1D and IS1F type. Zuber and Schumann (1993) identified the seventh IS1 copy at 49.6 minutes on the E. coli W3110 genetic map. Unfortunately, only the end 21-bp sequence as well as the neighboring 120-bp E. coli sequence were reported. We therefore designed two oligonucleotide primers to specifically amplify the seventh IS1 copy by PCR. One primer is homologous to the first sixteen bases of the IRL sequence of IS1A(IS1E), IS1B(IS1C) and IS1D. The other primer is complementary to the eighteen bases of E. coli sequence adjacent to IRR of the seventh IS1 copy. An 800-bp PCR fragment was obtained and its nt sequence determined, revealing an identical nucleotide sequence to that of IS1A(IS1E). A plasmid system was then used to isolate insertion mutations caused by insertions of the chromosomal insertion sequences. Of the 142 plasmid insertion mutants isolated, thirty-eight were insertions of chromosomal IS5, ten were IS30, and ninety-four were IS1. Detailed restriction mapping indicates that all ninety-four plasmid IS1 insertions were insertions of IS1 of the IS1A(IS1E) type.