Nitrogenase reductase (Fe-protein) was detected in the marine planktonic cyanobacterium Trichodesmium. The molecular weight was about 38 kD, as shown by western blotting using anti -Rhodospirillum rubrum nitrogenase reductase antiserum. The enzyme was confined to a limited number (ca. 10–40%) of randomly distributed trichomes in the Trichodesmium colonies, as shown by immunogold localization and transmission electron microscopy. Associated microorganisms had little or no nitrogenase. Nitrogenase showed a diel cycle in localization: present throughout the cytoplasm of cells in N2-fixing (daytime) colonies but at the periphery of non-N2-fixing (nighttime) colonies. This structural arrangement of N2-fixing trichomes and nitrogenase is novel and different from the previously held paradigm for this and other diazotrophic cyanobacteria.