Caseins are a family of milk proteins that exist in several molecular forms and are the main proteins present in the bovine milk. Genetic variants of these proteins have been associated with the quality and quantity of cheese derived from milk. Genotypes of 278 Frisón Negro Chileno cows were determined for <FONT FACE=Symbol>k</FONT>-casein by restriction fragment length polymorphism analysis of amplified DNA. A 350 bp fragment of the genomic bovine <FONT FACE=Symbol>k</FONT>-casein gene was amplified by PCR. Two HINF I sites were found in the amplified fragment of allele A, one at position 134 and one at position 266; only the latter site is present in allele B. Thus, digests of alleles A yielded 84 bp and 132/134 bp bands and digests of alleles B resulted in 84 bp and 266 bp bands. These bands, and thus genotypes AA, AB and BB, were recognised by agarose gel electrophoresis and ethidium-bromide staining. This technique was used to determine the <FONT FACE=Symbol>k</FONT>-casein allelic frequency in a Frisón Negro dairy herd. The distribution of genotypes was slightly different, and the gene frequencies similar to those reported in the literature. This molecular genetic technique based on molecular markers allows direct genotyping for milk <FONT FACE=Symbol>k</FONT>-casein with certainty and accuracy in bulls and females to be used in programs of dairy cattle improvement. Therefore, an early and precise identification of milk protein genotypes should have a direct impact on dairy cattle breeding strategies