The wood mouse is primarily a woodland species but it also occurs on maritime sand-dunes. The mice living on the sand-dunes leave their nests earlier in the evening and spend more time out of the nest than those animals living in woodlands. Here we test the hypothesis that this difference is because woodland provides substantially more food than do sand-dunes. Our experimental approach was to provide supplementary food in the form of wheatgrain to a sand-dune population and compare activity patterns with a control population. The mice on the supplemented area were strictly nocturnal and spent less time above ground than the controls. Their activity pattern was essentially the same as that of wood mice living in deciduous woodland.