Comparing food samples from diving and surface-feeding seabirds breeding in the Golfe du Morbihan at Kerguelen Islands to concurrent net samples caught within the predator foraging range, we evaluated the functional importance of the hyperiid amphipod Themisto gaudichaudii in the subantarctic pelagic ecosystem during the summer months. T. gaudichaudii occurred in high densities (up to 61 individuals m-3) in the water column, being more abundant within islands in the western part of the gulf than at open gulf and shelf stations. The amphipod was a major prey of all seabird species investigated except the South Georgian diving petrel, accounting for 39, 80, 68, 59 and 46% of the total number of prey of blue petrels, thin-billed prions, Antarctic prions, common diving petrels and southern rockhopper penguins, respectively. The length-frequency distribution of T. gaudichaudii was similar between the 2 diving species, which fed on 1 large size class of adult individuals, whereas the 3 surface-feeding seabirds preyed upon 2 size classes but in different proportions. Juveniles and adults T. gaudichaudii were equally important in the diet of blue petrels, whereas juveniles and adults predominated in the food of thin-billed and Antarctic prions, respectively. Comparison of T. gaudichaudii found in nets and food samples together with observations at sea indicated that common diving petrels and southern rockhopper penguins fed in the close vicinity of the colonies in the Golfe du Morbihan, whereas blue petrels, and thin-billed and Antarctic prions mainly preyed upon amphipods outside the sampled area. Our study shows that T. gaudichaudii is an important local component of the macrozooplankton community and the main prey for planktivorous seabirds inhabiting the Kerguelen archipelago. In certain areas of the subantarctic zone, it therefore has a trophic role similar to that of Antarctic krill Euphausia superba further south, in Antarctic waters.