Large particles (> 60 mm) were collected at 30 and 200 m water depth by in situ pumps in the southern Indian Ocean in January–February 1999. The samples were incubated under laboratory conditions with their own bacterial assemblages for 7–17 days in batches under oxic conditions in the dark. Particulate and dissolved fractions of organic carbon , amino acids, sugars and lipids, as well as bacterial production were quantified over time. During the experiments, 32–38% and 43–50% of total organic carbon (TOC) was mineralized and considered as labile material in the Polar Front Zone (PFZ) and Sub-Antarctic region (SAr), respectively. This material was utilized with a bacterial growth efficiency (BGE) of 10–21% (PFZ) and 12–17% (SAr), with the lower values being observed for surface samples (30 m). These results imply that most (79–90%) of the incorporated carbon from large particles was respired as CO 2. The study revealed that the initial relative abundance of the three main classes of organic matter, including sugars, amino acids and lipids, varied greatly between SAr and PFZ, with sugars being more abundant in SAr (15–19% of TOC) than in PFZ (8–9% of TOC). In the PFZ, mineralization rates of amino acids and lipids were two to ten fold higher than those of sugars, whereas the opposite was observed in SAr biodegradation experiments. Moreover, our results suggested that organic carbon is mineralized by bacteria more rapidly in the euphotic zone of the SAr than the PFZ. The differences observed between the two sites may be related to the more rapid dissolution of silica as well as the higher temperatures and bacterial production encountered in SAr waters. The bacterial processes apparently affect the composition of material sinking to the ocean interior. #