An anatomopathological study of the lesions produced by natural infection of Diphyllobothrium spp. in Oncorhynchus mykiss and Salvelinus fontinalis was carried out. Samples were collected from lakes and rivers of the Nahuel Huapi National Park and Reserve (Southern Argentina). Macroscopically, plerocercoid stages were seen either free, subperitoneal or encapsulated in various organs and musculature. The encapsulated plerocercoids were like nodules located on the surface or in the depth of the parenchymal organs. Microscopical studies showed that they corresponded. to granulomatous inflammatory reactions, in which intact parasites to cellular detritus with macrophage infiltration could be seen, depending on the time of the lesion. Based on our findings a possible pathogenic mechanism is suggested.