An assemblage of freshwater fishes inhabiting a medium‐sized stream in tropical central Vietnam was investigated with respect to spatial and trophic distribution. Point‐abundance sampling, gut content analysis and relative intestine length yielded interspecific differences in niche expression. Conspecific juvenile and adult habitat and feeding niches were also distinct. Niche differences arose mainly from differences in current velocity, substrata and foraging preferences. Extreme specializations, such as selective feeding on hard incustrations of cyanobacteria by adult Annamia normani, were present. The low niche overlap suggests that this tropical fish assemblage is structured by competitive interactions.