Using gel electrophoresis a genetic polymorphism of alpha S2-casein (Cn) was discovered in individual milk samples from 2 bovine breeds of the eastern part of France (Vosgienne and Montbéliarde). The 3 observed phenotypes (Plate 1) are determined by 2 co-dominant alleles at an autosomal locus. The alpha S2-Cn A variant was the only one known up to now in European breeds (reference variant) and alpha S2-Cn D is a new variant, whose bands overlap the beta-casein A band at pH 8.6, and migrate faster than alpha S2-Cn A at pH 3.0. The sequence of the polypeptide chain alpha S2-Cn D differs from that of alpha S2-Cn A by the deletion of a very acidic nonapeptide, which includes a cluster of 3 phosphoseryl residues. Due to the characteristics of the reference sequence, this deletion cannot be exactly located but it involves residues 50-58, or 51-59, or 52-60. A genetic analysis shows that locus alpha S2-Cn is closely linked to the cluster alpha S1-Cn--beta-Cn--kappa-Cn. The 4 casein species are thus synthesized by 4 closely linked loci.