Particle organic carbon flux from the shelf to oceanic waters during upwelling between 19¿N and 25¿N off Mauritania has been analyzed using coastal zone color scanner (CZCS) imagery. Biomass in the giant filament has been estimated using surface pigment concentration registered by CZCS during time series in December 1983, March 1984, and October 1984. Evidence for active phytoplankton growth in offshore waters is presented. Formation of the giant filament seems to be favored by the combination of a number of factors perhaps unique to the Mauritanian shelf. The large-scale circulation patterns promote offshore advection, and the typically long period of wind forcing can move the upwelling center to the shelf break area, thus causing maximal phytoplankton growth rate to be achieved in oceanic waters. The relatively high pigment concentrations registered by CZCS in oceanic waters suggest that nutrients have not been depleted on the shelf and that in situ growth is occurring in deep waters. We have used a baroclinic, hydrodynamic model to estimate cross-slope flux of particulate organic carbon at the 200-m isobath and derive a mean value of about 1¿105 g C s-1 during each of the CZCS time series. ¿ American Geophysical Union 1993