The occurrence of intestinal parasites was studies in adults and nestlings of Acrocephalus paludicola, A. schoenobaenus, A. palustris, A. scirpaceus and A. arundinaceus. The most common taxa in all these species were Coccidia and Ascaridia, and in some species also Ornithostrongylus and other Nematoda. In A. arundinaceus parasitized males were significantly heavier than nonparasitized ones, whereas in females the opposite was found. Moreover parasite-free males A. schoenobaenus and A. scirpaceus and females A. paludicola tended to be lighter compared to parasitized ones, but the difference was not significant. Infected A. scirpaceus females had significantly larger fat deposits than noninfected ones. Parasite prevalence also varied significantly between the promiscuous A. paludicola and the related monogamous species.