In this paper, we have described for the first time a semiquantitative method to evaluate histopathological damage, taking the degree of Neoechinorhynchus buttnerae attachment to the intestinal wall of the tambaqui (Colossoma macropomum), an important species in Brazilian aquaculture, into account. Twelve specimens of tambaqui were collected from a fish farm. Their bowels were removed and divided ... [Show full abstract] into seven morphologically distinct portions according to density and distribution of the parasite studies. Fragments from each fraction were histologically processed and analyzed. There was a clear preference on the part of N. buttnerae for the intermediate regions of the intestinal tube, where the highest densities were recorded. The intensity of damage to the host, estimated by calculating the Histopathological Alteration Index (HAI), showed severe and irreversible changes only where the parasite had its proboscis penetrated into the intestine wall.