We show that a family of prokaryotic repetitive sequences, called REP (repetitive extragenic palindromic), (Stern et al., 1984) is involved in the formation of chromosomal rearrangements such as duplications. The join-points of seven RecA+ tandem duplications previously characterized in Salmonella typhimurium, that fuse the hisD gene to distant foreign promoters, were cloned and sequenced. In all seven cases they are shown to have originated by recombination between distant REP sequences. Importantly, several join-points had also occurred at REP sequences even in a RecA-background. Thus, REPs can recombine with each other by a RecA(-)-independent mechanism involved in the generation of chromosomal rearrangements. While all RecA+ duplications analysed resulted from recombination between REP sequences, some RecA-duplications did occur also outside of REP sequences, in one case by recombination within a 7 bp homology. Possible roles for the known interaction between DNA gyrase and REP in chromosomal rearrangements are discussed.