Philureter trigoniopsis is the only parasite found in the ureters and urinary bladder of Galaxias maculatus in Patagonian Andean lakes. The dynamics of this endoparasitic monogenean were studied in Lake Gutiérrez, a body of water with scarce shoreline vegetation, where the host has an annual cycle of migration to the deep pelagic zone of the lake. To compare variations of the infection related to differences between lakes, G. maculatus specimens were sampled monthly with baited traps from September 1998 to November 1999 in Lake Moreno, which is an oligotrophic body of water with emergent shoreline vegetation and where the fish do not migrate to the deep pelagic areas of the lake. In addition, data for summer infections of P. trigoniopsis from 10 Andean Patagonian lakes that differ in aquatic vegetation, depth, and area were compared. In Lake Moreno prevalence of P. trigoniopsis showed a seasonal pattern, with 1-yr-old fish exhibiting the highest values of prevalence and mean intensity. Negative correlations between water temperature and prevalence and between age of fish and abundance were found. Our results suggest that age of fish may be the main factor structuring the distribution of P. trigoniosis in populations of G. maculatus. At the regional level the relationship between the infection and the characteristics of the lakes was also observed, with prevalence and mean intensity of P. trigoniopsis in G. maculatus higher in large deep lakes without macrophytes.