Summary.1Traps were set for the field mouse, Apodemus sylvaticus, in a great variety of environments on South Haven Peninsula in Dorset, an area already surveyed by ecologists.2A description is given of the areas chosen for trapping and the trap sites are shown on a map.3The numbers of mice caught in over 50 different environments are given in Table I.4Reasons why these numbers are not comparable are discussed.5Comparisons are nevertheless attempted between woods, dunes and heaths, and the woods are shown to have been the most populous.6Outside the woods the largest numbers of mice recorded in Table I were caught where the traps were set in a long line; this was probably because a line of traps drains a larger area than a cluster where other conditions are equal.7The same individuals were caught in several different environments.8In considering the preference of mice for different environments a method of census is needed which will discriminate between residents, visitors and mere passengers. The difficulty of finding such a method is emphasized.9A list is given in Table III of the distances between any two traps visited by the same mouse. Only one distance of over 400 yards is recorded, two between 350 and 400 yards, while below 250 yards the records become very frequent.10These distances do not show the length which a mouse can travel but result from the position of its nest relative to our arbitrarily chosen trapping sites. The longest distances are likely to have been recorded when the nest was centred between two sites.11The number of mice common to two areas depends not only on the distance between the areas but also on whether the intervening ground is habitable. Uninhabitable areas such as swamps are not so much barriers to mice as ground from which there are no mice wandering out in each direction.12Mice released at a distance from where they were caught found their way back over much greater distances than those recorded here for natural wandering. Longer wanderings observed elsewhere suggest that the mice may have been familiar with the country through which they returned, but this is hard to believe in the case of returns of over 600 yards from the opposite side of the lake.