The complement C3a anaphylatoxin receptor (C3aR) is a seven-transmembrane G-protein coupled chemoattractant receptor that on binding the C3a peptide ligand mediates numerous cellular responses, including histamine release from mast cells, smooth muscle contraction, and the directed migration of eosinophils. To delineate the murine C3aR coding sequence, gene structure, 5′-flanking region, and chromosome location, cDNA and genomic clones encoding the mouse C3a receptor were isolated, characterized, and used in fluorescence in situ hybridization experiments. The results from this study indicate that the murine C3a receptor structural gene is a single copy gene of approximately 8 kb comprised of 2 exons which are separated by a large intervening intron of 4724 bp. The first exon encodes 97 bp of 5′-untranslated sequence. Exon 2 encodes the remaining 8 bp of 5′-untranslated sequence and the entire coding and 3′-untranslated sequences. This genomic organization is typical of most other chemoattractant receptor genes in that the entire coding sequence is contained on a single exon. The human and mouse C3a receptor genes were localized to syntenic chromosomal bands 12q13.2 3 and 6F1, respectively. No other seven-transmembrane receptor genes, to date, have been localized to these chromosomal regions. Primer extension experiments using mouse macrophage RNA indicated a single transcriptional initiation site. Sequence analysis 5′ of the transcriptional site indicated a TATA-less promoter with possible cis-acting motifs that may regulate C3a receptor gene expression. These included the recognition sequence for the nuclear transcription factor SP1 and the phorbol ester response sequence which binds the Fos:Jun heteromeric transcription factor AP1.