Five Monogenea species were found on the gills of the catfish Pimelodus maculatus in Río de la Plata (Argentina). These were used for studying the preference of species on different gill-hemibranches, niche breadth and niche overlap between species. It was found that congeneric species had a generic-specific preference for certain gill-hemibranches. Niche breadth appeared to be related to the number of individuals of each species. Niche overlap between the species is discussed.