The Drosophila melanogaster homolog of the human proto-oncogene fos is Dfos. It is the only fos homolog in the Drosophila genome. Fos functions as a subunit of the heterodimeric transcription factor AP-1. There are two models of the Dfos gene. The first comes from a cDNA sequence of Dfos (Perkins et al., Genes Dev. 4 (1990) 822). The second is from the gene sequence published by the Drosophila genome project (Adams et al., Science 287 (2000) 2185), and there are notable contradictions between the two models. The promoter and the 5' end of the transcript sequence were not identified in either model. In this paper, we present the gene structure of Dfos and identify the promoter. This promoter has an initiator and a downstream promoter element sequence, but lacks a TATA box. Through comparison of the mRNA and genomic DNA sequences, three introns varying in length from 66 bp to 17.57 kb were found and verified by RT-PCR. The Dfos gene is 21.2 kb in length, giving a transcript of 3438 bp, coding for a predicted protein of 595 amino acids. The 3' untranslated region is confirmed to be 1092 bp in length.