Sandler (1970) suggested that mutation, abnormal oocyte (abo:2-38), may influence the function of the ribosomal RNA cistrons. We have examined the abo mutation and its interaction with the ribosomal DNA of Drosophila melanogaster. We observed that the expression of the abo phenotype is unstable under the appropriate conditions, a behavior which paralleled changes in the phenotypic expression of bobbed mutations during the magnification of the ribosomal DNA. The change in the expression of the abo phenotype is correlated with an increase in the redundancy of the ribosomal cistrons, further suggesting a functional interaction of the abo and bobbed regions.