The harlequin color pattern of Great Dane dogs differs from merle in that the background is white instead of blue. Harlequin by harlequin matings produced 60 black, 77 harlequin, 42 merle, and 35 homozygous merle pups. Harlequin by black matings produced 44 black, 26 harlequin, 25 merle, and one white (homozygous merle?) pups. All harlequins produced some merles. These data best fit the hypothesis that harlequin is a modification of merle (Mm) caused by an autosomal dominant mutation that is lethal to homozygotes, and to about half of heterozygotes when combined with the MM genotype. The symbol H is proposed for this mutation.