Is abnormal regulation of phaeomelanin synthesis, which results in yellow coat color, inextricably linked to the development of obesity in viable yellow (A ^vy) house mice? To answer this question, black male mice of genotypes A ^vy/A e^so/e⁺ and A/a^e e^so/e⁺ were produced from (DFT/Wf × C3H-A^vyfB/HeWf) × SO/LeWf matings. The sombre (e^so) mutation prevents phaeomelanin synthesis; therefore, all mice carrying this dominant allele are black regardless of the alleles present at the agouti locus. These black males were weighed weekly to 18 weeks of age. Simultaneously the agouti locus genotype of each male was determined by test matings to a/a e⁺/ e⁺ females. The results of these test matings indicated that the two genotypes could be classified rather accurately by the intensity of pigmentation of the belly hair. Black A^vy mice tended to have more dilute belly pigmentation than black A/a^e mice. These gross observations were confirmed by microscopic examination of several hair types. Black A^vy males reached a mean body weight of 44.2 ± 0.6 g at 18 weeks, while black A males had a mean weight of 30.7 ± 0.4 g at the same age. It is concluded that excessive weight gain resulting in obesity is induced by the A ^vy gene even when phaeomelanin synthesis is completely inhibited.