New Caledonia's unique flora evolved in the absence of ruminant herbivores until the 19th century. Native sclerophyll forests have been drastically reduced in extent, and browsing by introduced rusa deer is considered a major threat. We carried out a radio-tracking study on wild rusa deer on a mosaic of savannahs and sclerophyll forest. The estimated mean home range size was 501 33 ha, with little seasonal variation. Deer showed a remarkable site fidelity (the average distance between the site of capture and the geographic centre of activity was 1042 + 149 m). They used preferentially the sclerophyll forest and flood plains, especially during the dry season. These results confirm that rusa deer are likely to have a significant impact on the threatened sclerophyll forests, and they give some positive prospects for population control implemented locally.