Premunition is the state in a disease where an existing infection protects the host from reinfection with the same species. The cause of premunition is not clearly understood. In this study, we hypothesized that kin-selection might be a contributing factor in premunition. To test this theory, sheep were infected either once with a linguiform or smooth vulval morphotype of Haemonchos contortus, twice with the same morphotype or twice with different morphotypes. All infections resulted in a similar number of adult parasites. However, there were differences in the morphotypes recovered providing potential evidence of kin selection. Negative interference competition might also contribute to the reduction of the incoming population. Allelopathic or physical interactions between the parasites may be the mechanism behind the observed phenomena.