The heterotrophic assimilation of extracellular organic carbon (EOC) released by Littorella uniflora (L.) Aschers. was assayed. Utilization and decompostion of EOC by heterotrophs in the periphyton community were measured in a two-chamber open flow-system and, by the bacterioplankton, in bottle incubations. Under the experimental conditions 12–30% of the released EOC was metabolized in the periphyton community. This corresponded to a theoretical removal rate of 72–100% h −1. The bacterioplankton assimilated EOC more slowly (1% h −1). These results show that the EOC (mainly small molecules < 700 Daltons) was highly labile and accessible to heterotrophic microorganisms. The EOC released by submerged macrophytes in Lake Kalgaard is distributed approximately 20% to the periphyton community, 20% to the bacterioplankton, and 60% to the sediment.