Capping protein (CP), a heterodimer of α and β subunits, is found in all eukaryotes. CP binds to the barbed ends of actin filaments in vitro and controls actin assembly and cell motility in vivo. Vertebrates have three α isoforms (α1, α2, α3) produced from different genes, whereas lower organisms have only one gene and one isoform. We isolated genomic clones corresponding to the α subunits of mouse CP and found three α1 genes, two of which are pseudogenes, and a single gene for both α2 and α3. Their chromosomal locations were identified by interspecies backcross mapping. The α1 gene (Cappa1) mapped to Chromosome 3 between D3Mit11 and D3Mit13. The α1 pseudogenes (Cappa1-ps1 and Cappa1-ps2) mapped to Chromosomes 1 and 9, respectively. The α2 gene (Cappa2) mapped to Chromosome 6 near Ptn. The α3 gene (Cappa3) also mapped to Chromosome 6, approximately 68 cM distal from Cappa2 near Kras2. One mouse mutation, de, maps in the vicinity of the α1 gene. No known mouse mutations map to regions near the α2 or α3 genes.