Key words: Diet, Neotropical region, Lagoa Mirim system, Siluriformes.Abstract. The paper describes the trophic ecology of the Loricariichthys anus in a subtropical reservoir. The specimens were collected in the reservoir of the Chasqueiro stream, in Rio Grande do Sul, Brazil. The digestive tract of each specimen was removed for the analysis of the contents and definition of the diet. The Index of Relative Importance (IRI) was used to estimate which types of prey were most important. The diet of the L. anus population was compared among different water levels, seasons, ontogenetic classes, feeding strategy and fractional trophic level. There was a significant difference between hydrological levels, although no significant variation was found in the composition of the L. anus diet between seasons or among ontogenetic classes. This is probably due to the fact that L. anus is a specialist species, with a diet composed predominantly of fine particulate material. At the trophic level, the diet of the species was based on detritus, which reflects the morphology of its body and digestive tract. Considering these findings, L. anus may be a keystone species in the Chasqueiro reservoir and its conservation, in both natural and artificial environments, is necessary for the bottom-up management of this and other Neotropical ecosystems.