This study provides extensive data on planktonic N(2) fixation rates across the whole Mediterranean Sea. They show that N(2) fixation occurs in Mediterranean waters during the stratification period, with a clear decreasing trend from the oligotrophic western basin (10-76 mu mol m(-2) d(-1)) to the ultra oligotrophic eastern basin (0-0.4 mu mol m(-2) d(-1)). Highest rates are measured in the less oligotrophic western basin, between the surface and 75 m-depth, where 45 to 75% of N(2) fixation are found within the picoplanktonic fraction (<3 mu m). While the biogeochemical impact of N(2) fixation in the eastern basin seems negligible, N(2) fixation is able to sustain up to 35% of new primary production during the stratified period in the western basin. These data disagree with indirect estimates of N(2) fixation based on geochemical tracers and nutrient budgets, which indicates that N(2) fixation increases with increasing N:P ratios and decreasing stable N isotopic signature of particulate organic nitrogen and NO(3)(-) from west to east. These results finally point out the need to assess N(2) fixation at a higher temporal resolution in order to better understand the diazotrophs' dynamic under contrasted biogeochemical conditions.