The release of soluble organic carbon by the phytoplankton in the trophogenic layer of Lake Geneva has been investigated by14C method from March to November 1984. The carbon excretion rate varied from 0.01 to 0.5 mg C·m−3·h−1 and depended primarily upon the rate of its photosynthetic assimilation. The carbon excretion rate varied generally between 2 and 5% of photoassimilated carbon and exceptionally exceeded 6%. These values were underestimated from part of heterotrophic reassimilation. The study of the carbon balance over 24 hours has shown that night excretion represents less than 10% of losses due to respiration. Heterotrophic activity measured by14C glucose assimilation exhibited seasonal variations following the same general pattern as excretion. The quantity of phytoplankton excretion products appeared to be superior to the needs in organic compounds of heterotrophic organisms in the trophogenic layer.