The variations in the parasite population of fishes may be indicative of environmental alterations. With the aim of studying the variation of the parasitism by Ancyrocephalinae in Geophagus brasiliensis, two reservoirs of distinct trophic characterizations (Juqueri - eutrophic and Jaguari - oligomesotrophic) were compared. The prevalence rates and intensity of infection of this parasite were measured bimonthly for one year. The results showed that the prevalence and the intensity of infection vary in according to the alteration in the storage level of the reservoirs and consequently to the variations in the amount of material in suspension in the water.