The Deformed gene of Drosophila is necessary for the proper development of epidermal pattern elements arising from the maxillary and mandibular segments of the head. We find one major transcript (2.8 kb) homologous to Deformed (Dfd) probes which is expressed continuously from 3 h of embryogenesis into adulthood. Localized transcript accumulation is first detected just prior to the formation of the cellular blastoderm in a single circumferential band at about 65-75% egg length. The zone of Dfd expression is approximately two segment primordia in width. At later stages of embryogenesis, Dfd transcripts accumulate in the posterior ectoderm of the mandibular segment, and in the ventro-lateral ectoderm of the maxillary segment. Transcripts are also detected in the mesoderm and neuromeres of the mandibular and maxillary segments. The distribution of Dfd transcripts supports the hypothesis that Dfd functions as a homeotic selector gene in the determination of posterior head segments.