The recessive black plumage mutation in the Japanese quail (Coturnix japonica) is controlled by an amosomal recessive gene (rb) and displays a blackish-brown phenotype in the recessive homozygous state (rb/rb). A similar black coat color phenotype in nonagouti mice is caused by an amosomal recessive mutation at the agouti locus. An allelism test showed that wild type and mutations for yellow, fawn-2, and recessive black in Japanese quail were multiple alleles (*N, *Y *F2, and *RB) at the same locus Y and that the dominance relationship was Y*F2 > Y*Y > Y*N > Y*RB. A deletion of 8 bases was found in the ASIP gene in the Y*RB allele, causing a frameshift that changed the last six amino acids, including a cysteine residue, and removed the normal stop codon. Since the cysteine residues at the C terminus are important for disulphide bond formation and tertiary structure of the agouti signaling protein, the deletion is expected to cause a dysfunction of ASIP as an antagonist of alpha-MSH in the Y*RB allele. This is the first evidence that the ASIP gene, known to be involved in coat color variation in mammals, is functional and has a similar effect on plumage color in birds.