Killer whale (Orcinus orca) and sperm whale (Physeter macrocephalus) interactions with longline fishing operations were recorded by CCAMLR observers between 2000 and 2002 at South Georgia (Subarea 48.3). Demersal longlines, targeting Patagonian toothfish (Dissostichus eleginoides), were deployed in depths of 169 to 2 150 m. Most effort was concentrated along the 1 000 m depth contour. Sperm whales were the most abundant marine mammal observed in the vicinity of vessels when lines were being hauled, being present during 24% of hauling observations. Killer whales, the second most frequently sighted cetacean, were present during 5% of haul observations. A high inter-vessel variation was noted for interactions with both species. A comparison of geographic plots of cetacean sightings during hauls to fi shing positions showed that interactions occurred over a wide geographic range. These were mostly correlated to the fishing effort on the different grounds, although some ‘hotspots’ for interactions were noted. Killer whale pods were generally small (2–8 animals), while solitary animals and larger pods (>15 animals) occurred less frequently. Sperm whales were most often solitary when interacting with fishing vessels, although smaller groups (2–3 animals) were also relatively common. Interactions with killer whales were most often observed in the day, generally 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 when killer whales were present when compared to hauls during which no cetaceans were present. Catch rates were slightly higher in the presence of sperm whales and it is possible that sperm whales were attracted to some areas because of abundant prey (toothfish). However, in areas with lower catch rates, indications are that depredation by sperm whales can lead to a decrease in catches. Some mitigation measures have been tried by vessels to reduce interactions with cetaceans, although no quantitative studies have been carried out to measure their effectiveness. Apart from the obvious economic implications of fish loss due to depredation, ecological implications such as the effect of unrecorded fish removals on stock assessment models, modifications in the behaviour of marine mammals and entanglements with fishing gear are also important considerations. Further investigations are needed to determine the extent and effects of longline–cetacean interactions, to enable observer protocols to be standardised so as to ensure the collection of valuable data, and to assess and implement mitigation strategies under controlled experimental conditions.