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1. Despite widespread recognition of the role of body size in fish trophic ecology, little attention has been focused on this issue in isotopic studies, particularly in tropical systems.

2. We used analyses of stomach contents and stable isotopes to examine size‐related shifts in diet in a terapontid fish assemblage in the Australian wet–dry tropics. Stomach content analysis identified substantial ontogenetic dietary shifts in all species, corresponding to changes in body size–isotope trajectories for two species. Shifts away from relatively specialised diets of heavily ¹³C‐depleted insect larvae to consumption of a range of items across multiple basal carbon sources appeared to be the proximate cause of observed isotopic changes.

3. Allochthonous organic matter in the form of C3 riparian vegetation was particularly important to smaller terapontids before larger fish shifted to a broad range of dietary items and similarly …