Grey-headed mollymawks Thalassarche chrysostoma are killed by longline fishing operations in the southern Indian Ocean. We studied the foraging ecology of breeding grey-headed mollymawks at Marion Island, by tracking their foraging trips and sampling their diets. During the incubation period, birds made longer foraging trips, mostly towards the subtropical convergence and Subantarctic zone. This brought them into contact with areas of intense Southern Blue-fin Tuna Thunnus maccoyii longline fishing. During the early post-guard phase, foraging trips were shorter and to the southwest of the island, in the Polar frontal and Antarctic zones. Short foraging trips (<2 days) during this period, were made within the area of known Patagonian toothfish Dissostichus eleginoides longline fisheries around Marion Island. Diet samples during this period revealed a predominance of species associated with the Polar Front and waters to the south of it. The presence of the ommastrephid squid Martialia hyadesi in this study, is in contrast to previous studies at Marion Island and is of conservation importance due the potential commercial exploitation of this squid. Female grey-headed mollymawks spent a higher proportion of their time in areas of intense SBT longline fishing during the incubation period, while males spent more time within the boundaries of Patagonian toothfish sets during the early post guard phase. This may account for the male-biased mortality of grey-headed mollymawks observed in the toothfish fishery around Marion Island.