The bacteriophage P1-related plasmid p15B of E. coli 15T- contains a 3.5 kb long region which frequently undergoes complex rearrangements by DNA inversion. Site-specific recombination mediated by the Min DNA invertase occurs at six crossover sites and it eventually results in a population of 240 isomeric configurations of this region. We have determined 8.3-kb sequences of the invertible DNA and its flanking regions. The result explains how DNA inversion fuses variable 3' parts to a constant 5' part, thereby alternatively assembling one out of six different open reading frames (ORF). The resulting variable gene has a coding capacity of between 739 and 762 amino acids. A large portion of its constant part is composed of repeated sequences. The p15B sequences in front of the variable fusion gene encode a small ORF and a phage-specific late promoter and are highly homologous to P1 DNA. Adjacent to the DNA invertase gene min, we have found a truncated 5' region of a DNA invertase gene termed psi cin which is highly homologous to the phage P1 cin gene. Its recombinational enhancer segment is inactive, but it can be activated by the substitution of two nucleotides.