Clustered repeat units produced by BamHI digestion of genomic DNA from the Japanese monkey Macaca fuscata [JMr(BamHI)] were sequenced by dideoxy DNA sequencing. The nucleotide sequences of several individual repeats showed that the BamHI repeat contains the 170-bp HindIII element as an integral part, and that it has more than 90% homology with the HindIII repeat element [AGMr(HindIII)] found in the genomic DNA of the African green monkey. In the JMr(BamHI) repeat unit, the 170-bp HindIII element is flanked by a 6-bp inverted repeat, which is part of a 22-bp direct repeat. This latter repeat of 22-bp asymmetrically overlaps the border between the internal AGMr(HindIII)-like region and adjacent regions of the JMr(BamHI) repeat. A similar structural feature of the BamHI repeat unit has been found in the genomic DNA of the baboon, but not in that of the African green monkey. These results show clearly that the BamHI repeat of the modern Japanese monkey originated as a result of insertion of an AGMr(HindIII) element into a certain site(s) of the genomic DNA of an ancestor of the modern Japanese monkey before Macaca-Cercocebus divergence.