We describe the histopathological damage caused by parasite infection in farmed specimens of matrinxã Brycon cephalus, an economic important fish for aquaculture in the Amazon region. The fish were collected from an extensive fish farm, nearby the city of Nauta, Department of Loreto, Peru. Parasitic nematodes were found infecting the small intestine in 73 of 80 specimens (91.25%) of B. cephalus. According to morphological features of the examined specimens using light and electronic microscopy, the nematode was identified as Procamallanus (Spirocamallanus) inopinatus. Histological sections of the small intestine showed important damages on the tissue, such as desquamation, abrasion, compression, hypertrophy, and villi loss as well as necrosis in muscle layer, submucosa, and mucosa. This is the first report of tissue damage occurrences in the concerned host from fish farm in Peruvian Amazon, and it confirms the high pathogenicity of the nematode species and further points out the need of improving the strategies of parasitic prevention and control in order to better prevent future disease outbreaks that compromise production.