The relationship between heterotrophic nanoflagellates (HFLAG) and the autotrophic prokaryote Synechococcus was examined in the bay of Villefranche (NW Mediterranean) in June 1998. We determined Synechococcus concentrations, % of dividing Synechococcus cells, concentrations of HFLAG, and numbers of Synechococcus cells inside HFLAG food vacuoles in samples taken every 2-3 h over 62 h (n = 28). Synechococcus cells in division ranged from 10 to 30%, with peak values in late afternoon-early evening hours. Numbers of Synechococcus in HFLAG food vacuoles (0.06-0.34 Synechococcus HFLAG(-1)) were not related to concentrations df total Synechococcus, or nondividing Synechococcus but were negatively related to % of dividing Synechococcus. The data suggest there may be a relationship between vulnerability to predation and average Synechococcus cell size. HFLAG community grazing pressure, estimated from food vacuole content and HFLAG abundances, ranged from about 0.2% Synechococcus stock removed h(-1) at midnight to 1% Synechococcus stock removed h(-1) at noon.