Three aspects of the structure and expression of ribosomal RNA genes in Drosophila melanogaster have been studied. First, the expression of genes interrupted by type 1 insertions in bobbed mutant flies was studied; we found that these genes are not effectively expressed in the mutant. Second, we have obtained the primary sequence of the region around the initiation of transcription of rRNA precursors in several interrupted and uninterrupted genes. Third, we have studied the structure and expression of ribosomal RNA genes interrupted by type 2 insertions.