Resumo. Abstract Aim Studies of natural variations in fish diet allow, in turn, a better understanding of environmental changes along the hydrological cycle that can affect resources and, hence, biodiversity conservation. With this in mind, the present study aimed to understand how spatial and ontogenetic aspects (using Standard Length as proxy) define dietary composition, trophic position and trophic niche breadth for a small characid (Bryconamericus exodon) in streams located in two rivers of the Brazilian Pantanal. We also assessed whether spatial differences influence the structuring of trophic networks. Methods Fish were sampled monthly in the rainy season (October/2017 to March/2018) in four tributaries of the Negro and Apa Rivers, using different sampling methods. In the laboratory, fish were measured and weighed, followed by excision of stomach for posterior analysis. Results We analyzed 211 individuals, 126 from the Apa River (Standard lengthmin= 11.28mm; Standard lengthmax= 43.53mm) and 85 from the Negro River (Standard lengthmin= 13.26mm; Standard lengthmax= 40.05mm), that consumed mainly aquatic insects (Alimentary indexTotal= 87.97%), followed by terrestrial insects (Alimentary indexTotal= 9.02%). Dietary composition was mainly influenced by spatial variation (Pseudo-F1,194=12.21; p<0.001), followed by ontogenetic variation (Pseudo-F1,190=7.23; p<0.001), however, for trophic niche breadth, we did detect a higher importance of spatial variation (t=4.71; p<0.001) and an absence of ontogenetic variation (t=1.24; p=0.213). No spatial variation was detected for complementary specialization (p=0.998); only connectance showed a significant variation (p=0.047) with higher mean values in the Negro River (C= 0.27 ± 0.016) when compared to those of populations in the Apa River (C=0.22 ± 0.019). In addition, trophic position was not influenced by spatial (t= -1.77; p=0.077) or ontogenetic (t=0.69; p=0.494) variations. Conclusions B. exodon is considered an insectivorous species whose dietary composition can be explained more by spatial than ontogenetic variation.