Measurements of δ¹³C, δ¹⁵N, and C/N for a variety of Antarctic peninsula fauna and flora were used to quantify the importance of benthic brown algae to resident organisms and determine food web relationships among this diverse littoral fauna. δ¹³C values ranged from−16.8‰ for benthic algal herbivores (limpets) to −29.8‰ for the krill, Euphausia superba; the average pooled value for brown macroalgae, including their attached filamentous diatoms, was−20.6‰. There was no correlation between biomass δ¹³C or δ¹⁵N with C/N content, and consequently both δ¹³C and δ¹⁵N values were useful in evaluating trophic relationships. δ¹⁵N values of the fauna ranged from 3.1 to 12.5‰, with lowest values recorded in suspension feeders (e.g., bryozoans) and highest values in Adelie penguins (12.5‰) collected in 1989. The comparatively lower δ¹⁵N value for a Chinstrap penguin (6.9‰) collected in 1997 is attributed to the different dietary food sources consumed by these species as reflected in their respective δ¹³C values. Significant amounts of benthic macroalgal carbon is incorporated into the tissues of invertebrates and fishes that occupy up to four trophic levels. For many benthic and epibenthic species, including various crustaceans and molluscs, assimilation of benthic algal carbon through detrital pathways ranges from 30 to 100%. Consequently, the trophic importance of benthic brown algae may well extend to many pelagic organisms that are key prey species for birds, fishes, and marine mammals. These data support the hypothesis that benthic seaweeeds, together with their associated epiphytic diatoms, provide an important carbon source that is readily incorporated into Antarctic peninsula food webs.