Infective third-stage larvae of the cystidicolid nematode Pseudoproleptus sp. were found encapsulated in the mesentery of the freshwater fish Satanoperca jurupari Heckel (Cichlidae) from the Guamá River, close to the Amazon River Delta, Pará State, Brazil. The prevalence in fish (total body length 9-24 cm) examined from March 2009 to June 2010 (n=53) was 37%, with an intensity of 4-45 (mean 14±11) larvae per fish. The nematode larvae (body length 16.2-21.6mm), characterized by the cephalic end provided with a helmet-like cuticular structure having a thickened free posterior margin, were studied based on light and scanning electron microscopy. Fish play a role of paratenic hosts for this nematode species. This is the first record of a larval nematode of the genus Pseudoproleptus from fish, and the second record of a larval nematode belonging to Cystidicolidae in the Amazon and in the Neotropics. Additional larval specimens were found free in the stomach of only 0.9% Ageneiosus ucayalensis Castelnau (Auchenipteridae) examined (n=205). This finding can be considered as occasional parasitism.