Microbial activity in a marine sediment in Factory Cove, Signy Island (60°43′S, 45°38′W), South Orkney Islands in the maritime Antarctic was examined during December 1987 and January 1988. The sediment was bioturbated by a dense amphipod population in the surface layer but oxygen penetrated to a depth of only 1·7 mm. The top 1 cm was light coloured and contained negligible concentrations of acid-volatile sulphides. Below 1 cm the sediment was black and contained abundant sulphides. Sulphate reduction rates averaged 6·87 × 10−1 μmol sulphate cm−2d−1 over the 0–15 cm horizon, equivalent to 1·38 μmol organic carbon oxidized cm−2d−1. Of the sulphate reduced, 60% was to tin-reducible products (including pyrite) and 40% to acid-volatile sulphides. Annual sulphate reduction was at least 250 μmol sulphate cm−2y−1. The sea water temperature varied only between −1·8−1 °C, but the optimum temperature for sulphate reduction was 21 °C. Oxygen uptake by the benthos averaged 5·33 μmol oxygen cm−2d−1, equivalent to 5·33 μmol organic carbon oxidized cm−2d−1. Aerobic respiration accounted for 79% of the organic carbon mineralization and sulphate reduction for 21%.