The genetic basis for plumage color of the Blue Andalusian breed was studied. Results of crosses between Blue Andalusian females and Brown (eb/eb) tester males showed that this genetic stock was E/E and did not carry a columbian-type gene. This fact was further verified by the cross between Blue Andalusian males and Melanotic Prat (eWh/eWh Co/Co Ml/Ml) females. It is suggested that the Bl/bl+ genotype is effective in changing black to blue pigment when only one eumelanizing gene is present in the genetic background, but it is ineffective in the presence of two different genes producing eumelanin simultaneously. With an E/E genotype, Bl/bl+ does not change black pigment to blue in the areas where the melanotic (Ml) or lacing (Lg) genes produce black pigment, resulting in the laced plumage pattern of the Blue Andalusian (E/E Bl/bl+ Ml-Lg/Ml-Lg). On a non-E/E genetic background, a single dosage of Bl changes the black pigment to blue in the presence of the melanotic or lacing genes. Double-laced phenotypes were not found in either cross, thereby causing the authors to question the role of Co in double and single-laced patterns. Linkage between Ml and Lg was estimated to be 12.2% +/- 2.1 (SE).