We performed dilution experiments and primary production measurements in surface waters during a cruise in the northwestern Mediterranean Sea in June 2000 to quantify the carbon (C) flux through phytoplankton groups that can be distinguished by their pigment markers. Pigments in the dilution experiments were measured by high- pressure liquid chromatography (HPLC). Pigment per cell and synthesis rates of photoprotectant versus light-har- vesting pigments indicated that cells did not undergo photoacclimation during the 24-h dilution experiments. Mean chlorophyll a (Chl a) concentration was 0.128 mg m 23 , and mean primary production was 4.99 mg of C m 23 d 21 . Prymnesiophyceae contributed 51% of the total Chl a, green algae 21%, diatoms 13%, cyanobacteria 6%, and Pelagophyceae and Dinophyceae 4% each. Size fractionation showed that 68% of total phytoplankton were ,5 mm. A mean growth rate of 0.89 d 21 was completely offset by a mean grazing rate of 1.01 d 21 . Growth and mortality were also balanced for the individual phytoplankton groups, with highest rates for diatoms and lowest for green algae. Despite the high growth rates measured, nutrient-amended incubations showed that these rates were nutrient limited, although to a different extent for each group. Green algae experienced the most severe limitation and cyanobacteria the least. The comparison of C incorporation rates and Chl a synthesis rates permitted the estimation of a C : Chl a ratio of 47 (g : g) for newly produced organic matter in the study area. This value, corrected for the different degree of nutrient limitation, was used to estimate the C flow through the different phytoplankton groups. Because of their high growth rates, diatoms played a disproportionate role in the C flux compared to their biomass contribution.