The objective of the present study was to identify the Prolactin Receptor (PRLR) gene polymorphism related to litter size traits. The study included 101 Topigs 20 line of sows with 426 litters. The traits studied were: Total Number of Born (TNB), Number of Born Alive (NBA), Number of Still Born (NSB), and Number of MUMmifi ed (NMUM) piglets. Polymorphism was identifi ed with the polymerase chain reactionrestriction fragment length polymorphism method. Allelic and genotype frequencies and deviation from Hardy-Weinberg equilibrium were verifi ed with the chi-square test. Analysis of litter size traits was performed using the General Linear Model, which included the potential environmental effects. Additive and dominant allele variances were observed by the regression procedure. In the studied population of sows, the frequency of heterozygotes (0.5149) for PRLR gene exceeded the total number of AA (0.0198) and BB (0.4653) homozygotes, which resulted in a high proportion of B allele (0.7228). The results for PRLR showed statistically signifi cant (P<0.05) differences in fi rst parity sows between BB and AB genotypes for TNB and NBA. Signifi cant differences (P<0.05) were recorded in third parity sows between BB and AB genotypes for NBA, and in AA genotype versus AB and BB genotypes for NMUM. The fourth and subsequent parity sows of AA genotype had a signifi cantly higher (P<0.05) rate of NBA as compared with those of AB and BB genotypes. In all parities analysed, the difference between the BB and AB genotypes for NBA was statistically signifi cant (P<0.05). Interpretation of the results at the levels of phenotypes and either additive or dominant variance was quite diffi cult due to the small number of AA homozygous sows. The calculation model yielded a signifi cant effect (P<0.05) as well as tendency (P<0.1) for the mentioned effects except for age at fi rst farrowing.