1. Recently, a small predatory beetle, Trechisibus antarcticus (Carabidae), was accidentally introduced onto the island of South Georgia, sub-Antarctic. 2. From the presumed site of introduction the beetle is invading the coastal lowland area, building up high densities locally in the tussock-forming grass Parodiochloa flabellata. 3. In the coastal area the endemic detritivorous/herbivorous beetle Hydromedion sparsutum (Perimylopidae) is common, especially in and beneath the tussocks. 4. The first three, out of six, larval instars of H. sparsutum are easily taken prey by the carabid. 5. In sites colonized by the carabid, total abundance and the ratio between larval and adult numbers of H. sparsutum are far lower, and its adult body size clearly larger, than in comparable sites where the carabid is absent. 6. Two hypotheses are proposed for explaining the increase in adult body size of H. sparsutum: (i) the increase is a direct effect of predation: selection by the predator favours large hatchlings and/or larvae with a high growth rate; and (ii) the increase is an indirect effect of predation: by lowering the density of H. sparsutum, predation has increased its per capita food supply, enabling a higher growth rate and a larger adult body size. 7. A food addition experiment in a carabid-free site showed availability of high quality food to be insufficient for sustaining the initial larval population. 8. In the laboratory, females from the predator-infested sites produced larger eggs and hatchlings than females from the carabid-free sites, but mass specific growth rates of the larvae were not higher. 9. Field and laboratory data give stronger support to the food hypothesis than to the size selectivity hypothesis.