1] A combination of 15 N 2 labeling, Tyramide Signal Amplification –Fluorescent in Situ Hybridization (TSA-FISH) assay, and chemical analyses were performed along a trophic gradient (8000 km) in the equatorial Pacific. Nitrogen fixation rates were low (0.06 ± 0.02 to 2.8 ± 2.1 nmol L À1 d À1) in HNLC waters, higher in the warm pool (0.11 ± 0.0 to 18.2 ± 2.8 nmol L À1 d À1), and extremely high close to Papua New Guinea (38 ± 9 to 610 ± 46 nmol L À1 d À1). Rates attributed to the <10-mm fraction accounted for 74% of total activity. Both unicellular and filamentous diazotrophs were detected and reached 17 cells mL À1 and 1.85 trichome mL À1 . Unicellular diazotrophs were found to be free-living in <10-mm fraction or in association with mucilage, particles, or eukaryotes in the >10-mm fraction, leading to a possible overestimation of this fraction to total N 2 fixation. In oceanic waters, 98% of the unicellular diazotrophs were picoplanktonic. Finally, we found a clear longitudinal pattern of niche partitioning between diazotroph groups: while unicellular diazotrophs were present all along the transect, Trichodesmium spp. were detected only in coastal waters, where nitrogen fixation associated to both size fractions was greatly stimulated., Nitrogen fixation in the western equatorial Pacific: Rates, diazotrophic cyanobacterial size class distribution, and biogeochemical significance, Global Biogeochem. Cycles, 23, GB3012, doi:10.1029/2008GB003439.