Using size-fractionation filtration (1 μm), we associated carboxylase activities (Rubisco, β-carboxy-lases) and chlorophyll measurements with cell enumeration by flow cytometry at a permanent site of the central Ligurian Sea in the north-western Mediterranean Sea (73°25 1 N-7°51 'E). The analyses were carried out over a day/night cycle (at 30 m depth) following a strong wind event, during the transition period from spring mesotrophic to summer oligotrophic conditions. The highest values of Rubisco activity and β-carboxylase activity per chlorophyll a (Chl a) for >1 μm cells were observed during the light period of the cycle, reaching 18.9 and 4.3 nmol CO 2 (μg Chl a) -1 h -1 , respectively. This higher activity is assumed to be correlated with a dominance of nanoflagellates in the phytoplankton community Such phytoplankton species generally had higher β-carboxylase activity, expressed as a percentage of Rubisco activity (the βC/R ratio), than diatoms. Using flow cytometry analysis to enumerate those cells < 1 μm in size, we followed the values of Rubisco activity and pigment content expressed per cell, for picophytoplankton cells. The photoautotrophic activity, measured as the in vitro Rubisco activity for small picoeukaryote cells, was higher than for cyanobacteria cells with lower apparent cell size. These results suggested an optimum of CO 2 assimilation reached by the pico- and nano-phytoplankton in accordance with the cell size and growth rates from previous observations in the literature.