Nitrogen (N-2)-fixing microorganisms (diazotrophs) are an important source of biologically available fixed N in terrestrial and aquatic ecosystems and control the productivity of oligotrophic ocean ecosystems. We found that two major groups of unicellular N-2-fixing cyanobacteria (UCYN) have distinct spatial distributions that differ from those of Trichodesmium, the N-2-fixing cyanobacterium previously considered to be the most important contributor to open-ocean N-2 fixation. The distributions and activity of the two UCYN groups were separated as a function of depth, temperature, and water column density structure along an 8000-kilometer transect in the South Pacific Ocean. UCYN group A can be found at high abundances at substantially higher latitudes and deeper in subsurface ocean waters than Trichodesmium. These findings have implications for the geographic extent and magnitude of basin-scale oceanic N-2 fixation rates.