Resumo. Abstract Aim In the present work we describe and analyze the diet of a small characin species, Piabarchus stramineus, from the Ibicuí River, Uruguay River Basin, southern Brazil. Methods Samples were collected monthly from April 2001 to March 2002 with seine net. All fish were measured, weighed, and had its stomach extruded for gut content analysis. Months were grouped in seasons and fish were classified within three standard length classes (SLC). We tested for possible alimentary differences between the different seasons of the year and standard length classes. Results The analysis of the content in 301 stomachs identified 27 food items and low to intermediary niche breadth. The main food item/category was allochthonous insects, regardless of the seasons, and P. stramineus can be classified as an insectivorous species. We observed no food overlap between smaller and larger fish (SLC1 and SLC3). On the other hand, we observed food overlap between small and intermediary fish (SLC1 and SLC2) and between intermediary and large fish (SLC2 and SLC3), evidencing a transition in food consumption along size classes. A temporal variation in diet composition was also observed among size classes. Thus, different sized fish differed in their diets, with smaller fish feeding mainly on cladocerans (SLC1 and SLC2) and larger fish (SLC3) feeding mainly on allochthonous Diptera and Hymenoptera. Conclusions The species diet varies in time (seasons) and such variation is different in each size class. Allochthonous food items were important in the diet of the species in the different developmental phases, especially for larger fish, with autochthonous items also important, especially for smaller fish. The allochthonous food items are strongly related to the ciliary forest; thus, we emphasize the importance of the conservation of such environments.