Theoretical models predict that genetic relatedness affects the competition within and between parasite clonal groups sharing a common host. Here, we studied natural and experimental multiple infections of the trematode Coitocaecum parvum in its intermediate host. We focused on the effects of clonality on the life-history strategy of parasites competing for resources. Coitocaecum parvum can either delay maturation until its amphipod host is ingested by a definitive host, or adopt a progenetic strategy and reproduce inside the amphipod. Within a common host, clonal parasites were more likely to adopt identical life-history strategies than different genetic clones, both in natural and experimental infections. However, when timing of infection and other factors were controlled experimentally, parasites sharing a host were likely to adopt identical strategies regardless of their clonal identity, although pairs of clones were more likely to adopt progenesis than pairs of nonclones. The asymmetries in relative size and egg production between coinfecting parasites adopting the same life-history strategy were slightly, but not significantly, higher between different clones than identical clones. Our results suggest that the dynamics of competition between coinfecting parasites, although influenced by numerous external factors, is also modulated by genetic relatedness among parasites.