Nitrogen assimilation was measured in two austral summers in the Scotia Sea around the island of South Georgia as well as the Bransfield Strait. Nitrate and ammonium assimilation was measured using 15N techniques and the population was divided into two size classes, less than and greater than 20 μm. Water column integrated nitrogen assimilation rates varied between 2.43 and 26.50 mmol N m−2 day−1, the distribution being highly heterogeneous. The highest assimilation rate was found at a station near South Georgia, where the chlorophyll standing stock was elevated. A high assimilation rate was observed at a station in the Bransfield Strait and was associated with a localised, shallow mixed-layer feature. The less than 20 μm size fraction contributed to a variable but frequently significant proportion of the total assimilation (14–78%). f ratios were generally low, signifying a high dependence of the population on ammonium as a nitrogen source. Small phytoplankton exhibited a statistically significant greater preference for ammonium than large species, and the total community f ratio was influenced strongly by the proportion of the less than 20 μm fraction and ammonium concentration. It is suggested that nutrients may play a more important role in the ecology of the phytoplankton in the Southern Ocean than is usually supposed.