The polymorphism of a (TA)n(CA)n repeat microsatellite present in the third intron of the bovine kappa-casein gene (CASK) has been investigated. The existence of six alleles differing only in the number of dinucleotide repeats has been established. A total of 330 animals belonging to nine different pure bred Bos taurus French breeds or to a cross-bred Bos taurus x Bos indicus population (Créole) were genotyped. The distribution of the microsatellite alleles was examined and clear breed differences were noted. Genotyping of animals by isoelectric focusing (IEF) or restriction fragment length polymorphism (RFLP) (TaqI) was performed, in order to examine the relationship of the microsatellite polymorphism to other previously described CASK polymorphisms, at the protein and DNA levels. Strong correlation was seen, indicating that evolution of the various polymorphisms was not independent, and nine CASK haplotypes were observed.