Ontogenetic and seasonal diet changes of the juveniles of the marine fish Micropogonias furnieri, a species inhabiting Bahía Blanca estuary (Argentina), were investigated. Two size-related dietary shifts, at 4·00 cm and 7·00 cm total length (LT), respectively, were found. Small juveniles (1·00–3·99 cm LT) ate mostly chaetognaths (Sagitta friderici); medium-sized juveniles (4·00–6·99 cm LT) fed intensively on mysid shrimps (mainly Neomysis americana) and polychaetes, whereas large juveniles (7·00–15·99 cm LT) fed almost exclusively on epibenthic crustaceans (primarily Peisos petrunkevitchi). Increased mouth gape was related to increased size spectrum and mean size of the most important prey items consumed by M. furnieri. Juveniles <510 cm LT ate prey items almost as large as their mouth opening, whereas juveniles >410 cm LT were capable of consuming larger prey items than those found in their stomachs, indicating that the maximum size of prey eaten was not constrained by mouth gape. Seasonal and selectivity dietary analyses showed that M. furnieri can be a highly opportunistic selective feeder. Juveniles relied on S. friderici and P. petrunkevitchi throughout the year, except in summer, when mysids abundance increased in the estuary. Electivity values also showed that prey size and prey relative abundance are important factors in prey selection mechanisms.