Special mechanisms for stimulating recombination among the nearly identical repeat units of certain multigene families may exist in order to maintain their sequence homogeneity. We have found evidence for such a recombination-stimulating activity in the tandemly repeated ribosomal RNA genes of yeast. A fragment of the yeast ribosomal DNA (rDNA), containing the 5S gene, nontranscribed spacer DNA, and part of the 25S gene, causes a localized stimulation of recombination when inserted at novel locations in the yeast genome. The rDNA fragment stimulates both interchromosomal and intrachromosomal mitotic recombination but not meiotic recombination. To stimulate mitotic recombination, the fragment must act on both copies of the recombining gene. Furthermore, the rDNA fragment stimulates exchange only when inserted with the 5S gene proximal to, and the 25S gene distal to, the recombining alleles.