The spatial and temporal distribution of RNA and protein encoded by the homeotic Sex combs reduced (Scr) gene were examined during Drosophila development. The gene products are present in the epidermis of both the labial and first thoracic segments as would be predicted from prior genetic studies. However, the pattern in the central nervous system (CNS) and mesoderm is further restricted; the major expression located in the labial neuromere of the CNS and the mesoderm of the first thoracic segment. The spatial restriction within the CNS is correlated with and may be due to a differential timing of expression in the labial and first thoracic ectoderm. The labial ectoderm accumulates the Scr RNA prior to segregation of the neuroblasts while expression in the first thoracic ectoderm occurs after neuroblast segregation. The protein is also observed in the subesophageal ganglia of both larvae and adults, as well as in the labial and first thoracic imaginal discs. Surprisingly, the protein is also present to a lesser extent in second and third thoracic leg discs.