In Drosophila, the Deformed (Dfd) and Sex combs reduced (Scr) genes determined the developmental pathways followed by the most anterior metameric units. Using in situ hybridization, we have monitored the spatial distributions of transcripts from these two genes. Dfd RNA accumulates in parasegments 0 and 1; Scr RNA accumulation shows a dynamic pattern spanning parasegments 2 and 3. The expression of Dfd and Scr seems to change from parasegmental to segmental during formation of the gnathal appendages. Both genes are transcribed during imaginal development: Dfd in a portion of the eye-antennal disc; Scr in the labial and prothoracic discs. In addition, we find Scr RNA in the adepithelial cells of all mesothoracic discs.