CD4 has a central role in thymocyte differentiation and cell-mediated immunity. We isolated and analyzed chicken CD4. The gene spans 11.5 kb and is composed of ten exons. The promoter is TATA-less and similar to the mouse and human CD4 promoters, with two transcription start sites as determined by 5'RACE analysis. In general the introns are short, although the 5'untranslated region includes a large intron of 5.6 kb containing binding sites of the putative CD4 silencer. The single-strand conformation polymorphism technique was used to identify a polymorphism to map the gene, which lies on chicken Chromosome 1 in a position showing conserved synteny to mouse and human. This is the first report describing the organization of CD4 from a non-mammalian species. The structure and localization of chicken CD4 and many sequence motifs important in its regulation have remained conserved during evolution.