In Black-and-White dairy cattle, the kappa-casein gene (CASK) appears in three o f the most common genotypes, AA , AB, and BB . Apart from the coding sequence, several point mutations within the CASK gene promoter have been found. In this report, the hypothesis on the relationship between A/B polymorphism within exon IV and the promoter of CASK gene with milk performance traits is verified. One hundred and twenty four Black-and-White cows held in one herd were genotyped for two polymorphisms called CASK (exon IV, A and B allele) and CASK-R (kappa-casein promoter, Dde I restriction site in position -385, P and M allele). Sixty-nine AA, 48 AB and 7 BB CASK cows, and 92 PP, 30 PM and 2 MM CASK-R cows were identified. Associations between single CASK genotypes, single CASK-R genotypes as well as intragenic haplotypes CASK/CASK-R and milk performance traits were investigated. Significant differences (P<0.01) were found only for milk protein percentage. Cows with the PM CASK-R genotype and AA/PM intragenic haplotype showed the highest protein percentage. This is the first report showing the associations between mutations with in the promoter of the CASK gene, intragenic CASK haplotypes, and milk protein percentage. Our results support the hypothesis suggested in the literature that mutations within regulatory sequences of the CASK gene may influence the expression of milk protein genes.