The objectives of this study, based on the ecomorphological principle that morphology influences resource use, are three-fold. The first objective was to examine ontogenetic shifts in diet and morphology within a dominant predatory species, ~iChanna gachua.~i . The second objective was to examine difference in feeding ecology and morphology amongst the dominant family of Cyprinidae. The final objective was to examine the morphological and dietary differences among common riffle-dwelling fishes. The diet of ~iC. gachua~i was examined in relation to mass, mouth and body morphology and intestine length to examine the corresponding changes in diet with morphology. Cluster analysis was performed on the prey-specific abundance values of the dietary items and yielded four size-classes based on diet. There was a shift from detritus and smaller benthic invertebrates in the diet of the first size-class, to a mainly piscivorous diet in the fourth size-class as well as a steep increase in mass at the onset of piscivory. No significant change in relative intestine length was found with ontogeny and the average relative intestine length value for ~iC. gachua~i indicated primarily carnivorous feeding habits throughout ontogeny. Ontogenetic changes in mouth dimensions account primarily for the shift in diet, however, the ontogenetic change in body morphology is of secondary importance. The diet of seven coexisting cyprinids was examined in conjunction with morphological characters of the mouth and body, in Kanchanaburi, Thailand. ~iRasbora caudimaculata, Osteochilus hasselti, Cyclocheilichthys apogon, Garra fuliginosa, Danio acrostomus, Systomus binotatus~i and ~iMystacoleucas marginatus~i were all found coexisting in riffle habitats. Overall dietary overlap was low and diets ranged from detritus and algae for ~iO. hasselti~i and ~iG. fuliginosa~i, through benthic invertebrates for ~iC. apogon, S. binotatus~i and ~iM. marginatus~i, to terrestrial invertebrates for ~iR. caudimaculata~i and ~iD. acrostomus~i. Principal component analysis (PCA) elucidated the clear separation of ~iO. hasselti ~iand~i G. fuliginosa~i, as well as differences in morphology of the other five species. Canonical correspondence analysis (CCA) described relationships between the morphological characters and the diets of the seven species. Morphological characters, in particular mouth height and position, body width and depth and digestive tract length, accounted for differences in diet in the seven Cyprinids, allowing them to coexist. Morphological characteristics were examined in tandem with intestine content analysis for five species off co-existing freshwater fish in Thailand. ~iRasbora caudimaculata, Schistura desmotes, Dermogenys pusillus, Xenentodon cancila ~iand~i Monopterus albus~i are all found in riffle habitats in Thai streams. ~iR. caudimaculata, S. desmotes ~iand~i D. pusillus~i feed predominantly on ephemeropterans, hymenopterans and dipterans. ~iX. cancila~i feeds predominantly on fish and larger aquatic invertebrates such as odonata and ~iM. albas~i feeds on detritus as well as invertebrate prey such as crustaceans and odonata. Intestine length, mouth height, mouth width, eye position and mouth orientation varied among all five species. Canonical analysis of discriminance of mouth height, width and intestine length showed a clear dispersion of species, which was supported by intestine content. Morphology, in particular differences in mouth dimensions, accounted for differences in diet among the five species, allowing them to co-exist