The invertible deoxyribonucleic acid (DNA) segment cloned from Salmonella sp. was radioactively labeled and used as a probe to search for homologous sequences by Southern hybridization. Only one copy of the invertible segment could be found on the Salmonella sp. genome. Partial sequence homology with the invertible region was detected in bacteriophage Mu and P1 DNA by low-stringency hybridization. Under these conditions, no homology was detected with Escherichia coli DNA. A strain of Salmonella sp. defective in phase variation carrying the vH2- allele was also analyzed by DNA-DNA hybridization. The results show that there is sequence divergence between diphasic and vH2- strains within the invertible sequence.