Total polysomal RNA and polyadenylated mRNA from third instar larvae, pupae, and adults of D. melanogaster were hybridized in vast excess to labeled single-copy DNA in order to measure the sequence complexity of each RNA population. Then, to measure the sequence homology between the populations, each was hybridized to DNA enriched for messenger coding sequences in third instar larvae and to DNA depleted of these sequences. Our results show that a similar number of genes, approximately 16,000, is expressed in larvae, pupae, and adults, and that only one-third of these is expressed as polyadenylated mRNA. Further, the composition of both polyadenylated and nonpolyadenylated mRNA classes is shown to change very little between these three stages of development. Finally, the head of adult Drosophila is shown to contain 11,000 RNA species, approximately 70% of the number contained in the entire adult. © 1981.