Killer whale (Orcinus orca) and sperm whale (Physeter macrocephalus) interactions with longline fishing operations were recorded by CCAMLR (Commission for the Conservation of Antarctic Marine Living Resources) observers between 2000 and 2002 at South Georgia (Subarea 48.3) in the Southeast Atlantic Ocean. Demersal longlines, targeting Patagonian toothfish (Dissostichus eleginoides), were deployed in depths of 300 to 2000m, concentrated along the 1000m contour. Sperm whales were the most abundant marine mammals observed in the vicinity of vessels when lines were being hauled, occurring during 24% of hauling observations. Killer whales, the second most sighted, occurred at 5% of haul observations. A high inter-vessel variation was noted for interactions with both species. Geographic plots of cetacean sightings during hauls were compared to fishing positions. Both killer whale and sperm whale interactions occurred over a wide geographic range and were mostly dependent on the extent of fishing effort on the different grounds, although some ‘hotspots’ for interactions seem to occur. Killer whale pods were generally small, (2 – 8 animals, 57% of observations), while solitary animals (13%) and larger pods (>15 animals, 8%) occurred less frequent. Sperm whales were most often solitary (43% of observations) when interacting with fishing vessels, although smaller groups (2 – 3) were also relatively common. Larger groups were not often sighted. Interactions with killer whales were most often observed in the day, mostly in the afternoon, while night time interactions were relatively few and usually occurred before midnight. Interactions with sperm whales followed a similar pattern occurring most often in the afternoon, while very few interactions were observed at night. Catch rates were significantly lower (P > 0.05) when killer whales were present (0.15 kg/hook; 21.5 fish/1000 hooks), when compared to hauls with no cetacean present (0.29 kg/hook; 48.5 fish/1000 hooks). The same trend was, however, not observed for catch rates when sperm whales were present during hauling (0.32 kg/hook; 51.9 fish/1000 hooks). Catch rates were in fact slightly higher in the presence of sperm whales when compared to lines with no cetacean presence. It is likely that sperm whales were attracted to areas with high catch rates, but in areas with lower catch rates indications are that depredation by sperm whales can lead to a drop-off in catches. During hook-line observations on a longliner in the 2001 season it was noted that toothfish lips on hooks were more prevalent when sperm whales were in the vicinity of the vessel, suggesting that whole fish might be ‘stripped’ off the line, leaving only the lips. This would further complicate the quantification of levels of depredation. In contrast depredation by killer whales was often characterized by the occurrence of damaged fish, with often only the heads on returning hooks, although lips were also sometimes observed when killer whales were in the vicinity. Some mitigation measures have been tried by vessels to reduce interactions with cetaceans, although no quantitative studies were done to measure their effectiveness. Further investigations are needed to determine the extent of longline-cetacean interactions, to address the problems of longlinecetacean depredation, to standardise observer protocols to ensure the collection of valuable data, and to assess and implement mitigation strategies under controlled experimental conditions.