Simple SummaryMilk protein polymorphisms, especially κ-casein and β-lactoglobulin, are essential for the dairy and cheese industries. However,BB CSN3is a more favorable polymorphism, as it is responsible for a higher casein content. Statistically significant differences (p≤ 0.05) in interpregnancy periods values were observed between cows with theBBkappa-casein genotype and cows with theAAgenotype—the former exhibited the shortest interpregnancy periods. Additionally, cows with theBBgenotype showed more favorable reproductive indices, i.e., the lowest age at first calving and the lowest insemination index. However, these values were not statistically significant. There was no significant influence of any of the analyzed kappa-casein and beta-lactoglobulin genotypes on the course of cows’ delivery.AbstractThis research sought to determine the effect of selected milk protein polymorphisms on the reproduction parameters of 598 black and white Polish Holstein–Friesian cattle. The analyzed genes were kappa-casein (CSN3) and beta-lactoglobulin (BLG). The following reproductive indexes were selected: the age at first calving, the interpregnancy period, the calving interval, and the insemination index. The influence of the identified genotypes on the course of parturition in cows was also analyzed. Source data were collected for each cow from the beginning of the herd life and reproduction to its culling from the herd or the end of its third lactation. Data on the age at first calving, the amount of semen portions for artificial insemination (insemination index), the interpregnancy period, and the calving interval for each cow were also collected. A contingency analysis was performed through contingency tables using a Pearson’s chi-squared test for eachCSN3andBLGgenotype. The results show that theBBgenotype of the kappa-casein gene was associated with the most favorable values of reproductive indicators. In the case of the calving interval, the values were significantly more favorable than those of other genotypes (p≤ 0.05). No effect of beta-lactoglobulin polymorphism on the analyzed reproductive indices was observed. On the other hand, in the case of the interpregnancy period, significant statistical differences were obtained between theAAandBBgenotypes. The analyzed kappa-casein and beta-lactoglobulin genotypes did not significantly influence the course of parturition in cows. To conclude, the genotype polymorphismBB CSN3is the most favorable for the performance of the cows in the examined herd.