Parasitological examinations and the investigation of polymorphism in locus DRB1, belonging to MHC class II, were conducted in a flock of Polish Heath Sheep in the years 1995-1998. The intensity of parasite infection of dams ranged from 0 to 24 000 eggs, whereas for lambs it reached up to 27 000 nematode eggs in 1 gram of faeces. In the years 1995-1997 nematode species dominant in the infection were H.contortus and T. colubriformis, while in 1998 the predominant was T. circumcincta. DNA analysis of the examined sheep showed the presence of 20 alleles in locus DRB1, established on the basis of the length of a fragment of this gene amplified using PCR (exon 2 and microsatellite in intron 2). The analysis of variance conducted for the FEC trait in genotypic groups showed a statistically significant (P<0.01) association between the genotype in locus DRB1 and number of nematode eggs in sheep faeces. The presence of a given DRB1 allele in the genotype of animals examined had significant effect on parasite resistance. Moreover, the significant association between the FEC value in allelic groups of paternal half-sibs and the segregation of paternal alleles was also significant (P<0.05).