In free-living animals sexual selection is a central force shaping the spatial distribution of individuals in a population as well as sexual size dimorphism. We studied the influence of sexual selection on spatial distribution, female-to-male body size ratio, and female mating success of acanthocephalans in a natural host population of Saimaa ringed seal (Phoca hispida saimensis) harbouring a single intestinal helminth species, Corynosoma magdaleni. The acanthocephalans were always found along the full length of the small intestine; however, the site selection varied among the individual seals according to the age of the infection. The distribution of male acanthocephalans was not random with respect to females, with larger males tending to aggregate around non-mated females. A higher proportion of C. magdaleni females had copulated in seals with relatively more male worms. Male–male competition for access to females can be intense in C. magdaleni infrapopulation and may select for large males. We found that the larger the infrapopulation size, the smaller the males compared to females. In addition, the greater the female bias in the infrapopulation, the smaller the testes of males. Our study shows that sexual selection may be an important determinant of spatial distribution, male body size and female mating success of C. magdaleni in Saimaa ringed seal.