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 NO3- 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.