The sequence arrangements of a number of independently isolated λdv plasmids have been mapped by the electron microscope heteroduplex method. Heteroduplexes were prepared between the λdv DNA and an appropriate λ DNA, or between a λ addition phage DNA (λdv inserted into λ) and an appropriate λ DNA. The sequences present always include parts of immλ, and genes O and P. λdv plasmids derived from phage that are not deleted in the nin5 region are found to be monomers or direct (head-to-tail) oligomers. A large fraction of the λdv mutants derived from phage that are deleted in the nin5 sequence (i.e. λnin5-) are found to contain inverted repeat sequences; that is, they carry a sequence and its complement in an inverted order on the same strand. Some of these mutants also have unique (non-repeated) sequences. The presence of inverted repeat sequences is recognized by the property that when a closed circular duplex is nicked, the strands caused to dissociate by the addition of NaOH, and the solution reneutralized, the complementary sequences on a single strand rapidly renature ("snap-back") to form a duplex region. For inverted repeat λdv plasmids, we observe that: (1) the unique sequences of a λdv are always to the left and/or the right on the linear λ map relative to the inverted repeat sequences; (2) the junction of a sequence and its inverse is at the far left and/or the far right of that sequence, never in the middle. These observations lead to the hypothesis that inverted repeat λdv plasmids are formed by joining the two parental strands to each other and the two progeny strands to each other at each of the replication forks of a bidirectionally replicating λ molecule, by illegitimate recombination and/or by abnormal replication, as illustrated in Figure 9. We also propose that non-inverted λdv plasmids are formed by illegitimate recombination joining two points, one at each replicating fork, on one of the parental strands (Fig. 8). © 1974 Academic Press Inc. (London) Limited.