Total Escherichia coli DNA from strain DK445 (which is CSH50 F-, R-, deletion lac and pro, lysogenized with lambda cIts857 Sam7 lac5: :Mu cI+) was denatured, reannealed, and observed by electron microscopy. The single-strand DNA lengths ranged from about 50 to 150 kilobases (kb). In some molecules a short duplex region with a single-stranded fork at each end was observed. The duplex lengths were 0.75 kb, 1.30 kb, 5.22 kb, 5.62 kb, which correspond to those of IS1; of IS2, IS3, or IS4; of the ribosomal RNA genes; and of the γδ sequence, respectively. Duplexes of 1.0 kb and 0.5 kb were also found. Most of the duplexes of 0.5, 0.75, 1.0 and 1.3 kb were observed as intramolecular stem-loop structures and were therefore interpreted to be sequence duplications in inverted order on the same DNA strand. The most frequent separations of the putative inverted insertion sequences were around 22 and 27.5 ± 1.5 kb. About 14% of the E. coli chromosome is estimated to be involved in the sequence arrangements that give rise to stem-loop structures upon denaturation and reannealing. The copy numbers of the putative insertion sequences and other elements that form the "stems" of the stem-loop structures are also estimated. © 1977.