Recombinant DNA procedures and the yeast transformation technique were used to insert the yeast gene LEU 2 (coding for beta-isopropylmalate dehydrogenase) into the tandem array of ribosomal DNA genes of the yeast Saccharomyces cerevisiae. These insertions were shown by genetic and physical techniques to be unstable in meiosis. The meiotic instability was found to be the result of a high level of unequal recombination between ribosomal DNA gene clusters located on sister chromatids.