THE utilization of dissolved organic compounds by heterotrophic microorganisms is an important process in the aquatic carbon cycle and in the functioning of aquatic ecosystems in general1. It has been hypothesized that the only significant uptake of naturally occurring dissolved organic compounds by plankton is carried out by heterotrophic bacteria and that competition by phytoplankton for these compounds is insignificant2. In this communication I present evidence which does not support this hypothesis and I also show the importance of heterotrophy in relation to the total metabolism of plankton communities in freshwater ecosystems.