The effects of food supplementation on aMastomys natalensispopulation were experimentally studied in a grassland habitat in Swaziland. Food was added bi-weekly to two 1 ha grids, while a single 1 ha grid served as the control. Mice were trapped monthly over a twelve month period. Food supplemented grids supported more than twice the number of mice than the control grid. This increase was a consequence of high rates of immigration onto the supplemented grids. Food supplementation also extended the breeding season of the females by at least two months, but not of the males. Late in the dry season (August—October) the mean mass of mice was higher on the supplemented grids. Monthly residency rates were higher on the supplemented grids immediately after supplementation (July—August), and during mid-summer (December—February). The presence of extra food, however, did not prevent the spring (October—November) decline.