The wet—forest stream fishes of Sri Lanka are segregated in ecological and morphological space. Suites of selected morphological attributes of the fishes were correlated with microhabitat and feeding ecology. Previous studies have shown that these fishes partition resources with pronounced complementarities along niche axes, a pattern consistent with assemblages structured by interspecific competition. Since such competitive interactions may have been significant historically, the speciations and extinctions that have occurred to produce the contemporary assemblages, since the early Pleistocene isolation of wet—forest fish fauna from the mainland fauna, most likely occurred in an arena of interspecific competition. Invasions of the Sri Lankan wet—forest streams by mainland fishes after the initial isolation of the island were also unlikely due to various biogeographic, climatic, and geologic barriers, Thus the data favor the hypothesis that the ecomorphological relationships exhibited by these fishes may be the results of evolutionary adjustments to facilitate resource partitioning, thereby reducing interspecific competition.