Phytoplankton release a variable fraction (0 to > 80%) of the photosynthetically fixed inorganic carbon as extracellular dissolved organic compounds. Despite this wide range and the potential effects on carbon fluxes and food p webs, the knowledge on how the environment and phytoplankton species involved influence this process is incomplete. Notably, there are no estimates for release of dissolved organic carbon by Prochlorococcus, a marine cyanobacterium estimated to be the numerically dominant oxygenic phototroph in the oceans. Here we report extracellular release of dissolved organic compounds from two axenic Prochlorococcus strains representing different ecotypes (MED4 and MIT9312) cultured under nutrient replete and phosphorus limited conditions. Independent assays based on C-14-bicarbonate tracers and analyses of particulate and dissolved organic carbon suggest that the release of dissolved organic carbon ranged from 9 to 24% of the total assimilated inorganic carbon with slightly lower values for phosphorus limited cultures. Between 4 and 20% of the released organic matter consisted of low molecular weight carboxylic acids, compounds known to be highly labile substrates for heterotrophic bacteria. In oligotrophic oceans where Prochlorococcus is a dominant contributor to primary production and input of terrigenous organic matter is negligible, this process is likely a significant contributor to the pool of organic substrates available for microbial heterotrophs.