Velocity sedimentation studies of RNA of Sarcophaga bullata show that the major rRNA species have sedimentation values of 26S and 18S. Analysis of the rRNA under denaturing conditions indicates that there is a hidden break centrally located in the 26S rRNA species. Saturation hybridization studies using total genomic DNA and rRNA show that 0.08% of the nuclear DNA is occupied by rRNA coding sequences and that the average repetition frequency of these coding sequences is approximately 144. The arrangement of the rRNA genes and their spacer sequences on long strands of purified rDNA was determined by the examination of the structure of rRNa:DNA hybrids in the electron microscope. Long DNA strands contain several gene sets (18S + 26S) with one repeat unit containing the following sequences in order given: (a) An 18S gene of length 2.12 kb, (b) an internal transcribed spacer of length 2.01 kb, which contains a short sequence that may code for a 5.8S rRNA, (c) A 26S gene of length 4.06 kb which, in 20% of the cases, contains an intron with an average length of 5.62 kb, and (d) an external spacer of average length of 9.23 kb.