|Beschreibung:||Parasite infections containing genetically different parasites of the same species are widespread in nature. The genetic diversity is predicted to influence parasite load, the evolution of virulence and host defence mechanisms. According to kin-selection theory, competition is stronger between individuals of different genotypes than between genetically identical individuals. This leads to a predicted lower parasite load in a diverse infection compared to a uniform infection. I studied the importance of genetically diverse infections for three-spined sticklebacks (Gasterosteus aculeatus) parasitised by the trematode Diplostomum pseudospathaceum. I showed that genetically diverse infections are widespread in natural populations. In experimental infections, I demonstrated that different genotypes infecting one host do indeed suppress each other as predicted by kin-selection theory, leading to a lower total parasite load in a mixed genotype infection compared to single genotype infections.