Abstract – The relationship between ontogenetic changes and both feeding and habitat preferences was studied in a stream catfish population of Argentine Patagonia. Fish capture, as well as habitat data recording, was performed during a 1-year long period. Larvae–juvenile transition was determined between 22.4 and 29.4 mm standard length (SL), on the basis of the relative growth changes and morphogenesis, and juvenile–adult shift was established between 61 and 65 mm SL using a macroscopic criterion of gonadal development and gonadosomatic index. Spawning period was estimated to be in the summer season, from December to February. Larvae preferred shallow marginal pools and fed mainly on small Chironomidae larvae, while juveniles and adults inhabited riffles and preyed on Ephemeroptera nymphs and Chironomid larvae. Morphological constraints during the larval period were related to habitat and feeding preferences. A complete development of fins allowed juveniles to colonise faster water habitats while a bigger mouth gape permitted them to prey on new items and on a larger size prey range.