The Diptera are one of the dominant insect consumer groups on sub- Antarctic islands and are thought to contribute significantly to terrestrial ecological processes at many of these islands. The life-cycle of Paractora trichosterna and its contribution to kelp degradation at Husvik Harbour. South Georgia were therefore investigated in the laboratory and in two artificial wrack beds in the field. Duration of the larval stage was approximately 2 months at 10 C, during which time larvae attained a maximum individual mass of ca. 90 mg. Larvae had a relative consumption rate of 0.734 mg dry mass kelp mg dry mass larva-1 day-1 Based on this rate and information on larval densities of P. trichosterna, and a smaller species. Antrops truncipennis, kelp consumption was estimated to be 714- 870 g dry mass kelp m-2 over the 7-week study period. During this time, kelp dry biomass declined to 30% of its original value, both in a bed protected from trampling by vertebrates and in an exposed one. P. trichosterna was directly responsible for 12% of this loss in the protected bed and 20% in the exposed one. A. truncipennis was responsible for an additional 3% loss in the exposed bed and 8% in the protected one. These fly species therefore contributed significantly to kelp degradation. Differences in biomass of the larvae and adults of the two species between the beds suggested that P. trichosterna prefers more exposed wrack than does A. truncipennis.