Marine planktonic primary production was studied in Borge Bay, Signy Island, the Antarctic, during the period January 1972–February 1974. Productivity was measured in situ by the radiocarbon uptake method. In 1972–1973 and 1973–1974, respectively, 86 and 289 gC m-2 a-1 was fixed in 16.5 m of water. The seasonal production cycle consisted of a rapid increase of a diatom standing crop dominated by Thalassiosira antarctica Comber, to a peak crop of 0.381-1 m-2 standing cell volume, 605 mg chla m-2 and 23.5 gC m -2, by the first week of January. Mid-summer productivity was high, 4.8 gC m-2 d-1 being fixed on occasion but specific fixation rates were relatively low (maximum 1.12 mgC mg-1 chla h-1). In late February the standing crop declined abruptly and during the winter period productivity and standing crop was small. Productivity and standing crop was greatly influenced by light climate and water stability.