Data from the Mediprod 1 cruise (March-April 1969), together with the appropriate conversion factors, were used to calculate carbon flux in the pelagic ecosystem, following fertilization due to vertical winter mixing. In the space of one month, 13.7 gC multiplied by m super(-2) are produced by the phytoplankton, one quarter (3.4 gC multiplied by m super(-2)) of which corresponds to a rapid regeneration of nutrients within the euphotic layer mainly by the way of zooplankton. One quarter of the production (3.4 gC multiplied by m super(-2)) is used by the phytoplankton to increase its biomass. The remainder of the photosynthetic carbon (10.3 gC multiplied by m super(-2)) is exported outside the primary producers box. About one half (5.2 gC multiplied by m super(-2)) is used for maintenance and growth of the zooplankton which increase their production tenfold. However, during this period, the biomass of the zooplankton increases by only 1 gC multiplied by m super(-2), which indicates high metabolic requirements and (or) rapid transfer to predators. Finally, about one half leaves the system either to deep layers or to the periphery by gliding along isopycnal surfaces.