Samples of algae, epilithon, tree leaves, aquatic invertebrates, and feathers of blue duck{Hymenolaimus malacorhynchos)were collected for stable carbon isotope analysis from Manganuiateao River, central North Island, New Zealand, to elucidate the diet of blue duck and the carbon pathways involved. 513C values for leaves were typical of terrestrial vegetation elsewhere (‐27 to ‐31%o), but, in contrast to other values found in New Zealand, algae were much richer in13C (> ‐17.0%c) than terrestrial vegetation. Larvae of the caddisfliesOlinga feredayiandAoteapsyche colonica, and the dipteranAphrophila neozelandicawere usually enriched with13C (mostly > ‐20.0%c) relative to other taxa, indicating mat they had been assimilating higher proportions of algal‐derived carbon. In contrast, 813C values < ‐20.0%e were recorded for the mayfliesDeleatidiumspp. andColoburiscus humeraUs, and the caddisflyBeraeoptera roria, indicating that their diets were composed of a higher proportion of organic matter of terrestrial origin. Feathers from blue ducks living in the lower part of the study section were enriched with13C (‐21.9 to ‐15.6‰) compared with those from the upper part of the study section (‐25.0