Extracellular release of phytoplankton was investigated in the upper middle River Meuse (Belgium) in order to estimate the contribution of this process as a source of dissolved organic matter in the system. Particulate primary production, exudation and reassimilation of exudates by free-living and attached bacteria were estimated in parallel using14C labelling and selective filtration techniques. The results showed that total phytoplanton exudation (EOCt) represented on average 7% of the particulate primary production. Of this excreted organic carbon, 77% was taken up by bacteria; attached bacteria contributed 30% of this uptake. Calculations showed that in this river system, the excretion of organic carbon by algae accounted on average for 22% of the bacterial carbon demand.