A computer search for repeated sequences led us to identify five REP (repetitive extragenic palindromic) sequences in the 3'-terminal region of the Escherichia coli ribonucleoside diphosphate reductase gene (nrdA). These REP sequences are located within a putative duplicated DNA region, the first of them being part of the carboxy-terminal coding region of the nrdA gene. This is the first report of a REP sequence within a structural gene and also the first example of a REP sequence apparently generated by DNA duplication.