Carbohydrate metabolism and heterotrophic carbon fixation have been studied in the intertidal macrophytic brown algae Pelvetia canalioulata and Fucus spiralis. In neither of these species can the heterotrophic metabolism of glucose support net growths. Photosynthetic rate, however, is enhanced by over 40% in Pelvetia in the presence of exogenous glucose. It is postulated that this glucose lowers the light requirement for photosynthesis rather than acting as a substrate for significant levels of heterotrophic carbon fixation.