Stable isotopes are employed in many fields of ecology. We are using a recently proposed approach to assess a species’ isotopic niche, which represents the area occupied by individuals in the δ-space – obtained by plotting δ¹⁵N against δ¹³C. As part of a NSF-FIBR project on feedbacks between evolution and ecosystem function, we assessed the isotopic niche of two fish species, Rivulus hartii and Poecilia reticulata, from three Trinidadian streams characterized by different predation and competion regimes. In each stream, two lacking their natural shrimp population, fishes were sampled from three different communities: Rivulus+Poecilia+other fish predators, Rivulus+Poecilia, and Rivulus only. Rivulus and Poecilia show niche overlap for small-sized individuals. There was no niche overlap in adults, probably due to resource partitioning. Ontogenetic niche shift was inferred using isotopic signatures for individuals of different size classes. In both Rivulus and Poecilia, the δ¹⁵N signals were positively correlated with size. δ¹³C signatures increased with size in Rivulus and decreased in Poecilia. Our initial findings confirm the utility of the isotopic niche approach to better understand community dynamics in tropical streams. We will continue to use this approach to examine hypotheses regarding how niche breadth is directly affected by different predation and competition regimes.