To study the effect of alphas(1)-Cn polymorphism on milk production traits, milk samples front Brown Swiss, Holstein and East Anatolian Red cows were analyzed for alphas(1)-Cn BB and BC types using horizontal starch-gel electrophoresis method. Proportion of alphas(1)-Cn BB and alphas(1)-Cn BC in terms of alphas(1)-Cn phenotype were 74.3% and 25.7%, respectively. No significant differences were found among breeds in frequencies of alphas(1)-Cn. phenotype. Although the effects of alphas(1)-Cn phenotypes on milk yield, milk fat yield, and 305-days milk yield in Holstein cows were found to be significant (P<0.05), no effect on these traits in Brown Swiss and lactation period, 305-days fat yield and fat percentage in both the breeds were observed.