The central aim of community ecology is to understand how the numbers and relative abundances of species in an assemblage are determined. Parasite communities in freshwater fish are often described as unpredictable, depauperate communities. However, monogenean species on the gills of cyprinid fish often form multispecies communities with even thousands of specimens on one gill-arch. The factors affecting parasite communities were studied at infra- and component community level in two cyprinid fish species; roach (Rutilus rutilus) and crucian carp (Carassius carassius). First, I discuss the environmental factors determining the numbers and species of parasites in the community. I focus first on host population structure, but environmental diversity, host immunity and physiology are also discussed. Recruitment of the parasite species onto the gills is another important theme not previously studied. Second, I concentrate on the factors affecting the site-preference of the parasite specimen on the gills. Competition between species is one possible factor determining site preference. However, no competition between species was detected in any of the communities studied, in spite of unusually high abundances. Aggregation of parasites on certain host specimens is one of the most peculiar phenomena of parasite communities. Here I show that it is one of the most prominent features also at the infracommunity level. Individuals are more aggregated at lower abundances, in order better to find a mate. Other possible factors determining the occurrence on the gill-arch are the size of the gill-arch and the direction of the water current through the gills. Finally, the level of the study and its usefulness in other freshwater fish parasite communities are discussed