After twenty-seven years of human absence on Hirta, the St. Kilda field-mouse population in the Village Glen was numorically unaffected over two years, by a large-scale military task-force operation. The movements of field-mice were experimentally examined. The trap revealed home range of a breeding male was about three-quarters of an acre. Mice transported distances of up to 1,600 feet were able to return to the trap-site in probably less than three hours. Fewer were able to do this over 2,800 feet. Mice were able to move up-and downhill with equal facility. More mice were caught in the hour during and after dusk, than during the remainder of the night. A detailed summary is given.