In the western Sargasso Sea in early autumn the standing crop of Oscillatoria thiebautii at nine stations averaged 640 × 105 cells m−2 with a maximum population of 11.8 × 105 cells m−3 at 25 m. Population density at 175 m averaged only 6% of that at 25 m. N2 fixation was greatest at 1‐m depth, 0.069 pg cell−1 h−1, and at 50 m decreased to 9% of that at 1 m. Highest recorded N2 fixation, 0.20 pg cell−1 h−1, was at the southernmost station. Average N2 fixed per square meter of sea surface in the upper 50 m was 2.0 µg N m−2 h−1. When compared with other N inputs to the surface waters of the western Sargasso Sea, N2 fixation by Oscillatoria seems negligible.Results of 15N‐labeled NH4+, NO3−, and urea uptake experiments indicate that O. thiebautii is barely able to utilize these nutrients at the concentrations present in the euphotic zone. Doubling times (Td) for cellular nitrogen by N2 fixation and NH4+ uptake together are in the order of 40 to 110 days. An uptake curve for NH4+ yielded a K. of 6.7 µg‐atoms N liter−1, a value about an order of magnitude greater than that found for other open‐ocean phytoplankters.We were unable to isolate any N2‐fixing bacteria from O. thiebautii.