The guillemot (Uria aalge) is a dimorphic species in the Atlantic part of its range with both a normal and a “bridled” form. The results of hand-rearing guillemots of known parentage are compatible with bridling being a recessive variant from the normal, carried by a single gene on one of the autosomes. The first definite signs of bridling were seen as early as 54–66 days after hatching. There was no significant difference between the bridled and normal variants in the body weight and wing length of breeding adults and egg weight and hatching weight of the young. The likelihood of physiological differences between the two variants and also the factors determining morph-ratio and its changes with time are discussed.