In three species of algae and three species of cyanobacteria excretion of dissolved organic carbon (DOC) during photosynthesis on H14C03 ranged from 0-03 to 0-39 pg C pg Chi a'1 h“Extracellular DOC from Anabaena cylindrica was assimilated by a Flavobacter sp. However, Lysobacter CP-1, an organism closely associated with bloom-forming cyanobacteria, increased the amount of DOC released by photosynthesizing Gloeotrichia echinulata and A. cylindrica. No similar effect was found with mixed cultures of Lysobacter CP-1 and Scenedesmus quadricauda. Lysobacter CP-1 grew in filtrates of A. cylindrica and S. quadricauda cultures predominantly at the expense of the following amino acids; aspartate, glutamate, isoleucine, glycine, histidine and serine/glutamine. The results presented here suggest that the metabolism of Lysobacter CP-1 is finely tuned to the extracellular DOC released by algae and cyanobacteria. Additionally Lysobacter CP-1 isolates may also be capable of uncoupling the rate of extracellular release from the photosynthetic rate. The net effect would be to increase the availability of algal derived DOC to heterotrophic organisms within the phytoplankton assemblage.