Two main genetic variants of κ-casein, A and B, are found in Holstein cows. A positive association between the κ-casein B genetic variant and milk characteristics of economic importance have been reported (reviews: Creamer & Harris, 1997; Ng-Kwai-Hang, 1998). This can be explained, at least in part, by a higher κ-casein content of the casein in milk of cows having the κ-casein B genetic variant (Donnelly et al. 1984; Law, 1993). We have recently reported the presence of a differential allele-specific expression of the κ-casein gene in Holstein cows by comparing the content of allele-A and allele-B specific mRNA in the mammocytes of lactating cows genotyped κ-casein AB (Robitaille & Petitclerc, 2000). For some cows, the A- and B-specific κ-casein gene alleles were expressed in a similar manner. These cows are named HH here to indicate that they were homozygous for κ-casein gene expression. The other group of cows, cows HL, presented a differential allele specific expression of the κ-casein gene allele B which was overexpressed compared to allele A. The genetic polymorphism governing this differential allele-specific expression of the κ-casein gene is likely to affect the physico-chemical properties of casein micelles and, hence, the technological properties of milk, as κ-casein is considered to be the stabilizing component of the milk micelles. The objective of the study was to evaluate the effect of this expression polymorphism of the κ-casein gene on milk ethanol stability.