dif (deletion induced filamentation) is a newly identified locus that lies within the terminus region of the Escherichia coli chromosome. The Dif phenotype was characterized by a subpopulation of filamentous cells with abnormal nucleoids and induction of the SOS repair system. Interactions between dif-carrying plasmids as well as between such plasmids and the bacterial chromosome demonstrated that dif is a cis-acting, recA-independent recombination site. Filamentation continued in dif mutants in which SOS-associated division inhibitors were inoperative, which showed that induction of these inhibitors was not the primary cause of filamentation. Filamentation was not observed in dif recA or dif recBC mutants, which were unable to carry out homologous recombination. The dif site shows homology with the cer site of plasmid ColE1, which resolves plasmid multimers to monomers. It is proposed that dif functions to resolve dimeric chromosomes produced by sister chromatid exchange, and that the Dif phenotype is due to the inability of these mutants to resolve multimers prior to cell division.