From the literature it is known that woodlice are badly equipped for life on land, but because they can generally be found in dense populations they must be able to maintain life in land habitats. In 1952 it was found that in the dune area "Meijendel" near Den Haag woodlice show remarkable nocturnal activities, and it was supposed that these activities had something to do with their adjustment to the habitat. In 1953 an investigation into the ecological significance of activity patterns in the woodlouse Porcellio scaber Latr. was started in two aspen woods in "Meijendel". It was found that the amount of different forms of activity and the quantitative relations between these forms are correlated with air humidity. The duration of the individual periods of activity depends on air humidity and is for the great majority of active specimens about one hour. The activity patterns of P. scaber have a distinct effect on the distribution of the animals over a wood (day-habitat) and contribute to an effective dispersal of the species. The suitability of a day-habitat for P. scaber depends on its moistness, since it must essentially be considered a shelter against the relative dryness of the day-period. Generally the best shelters are offered by the loose bark of dead or dying trees and by compact objects lying flat on the ground. The observations made under natural conditions may be generally understood by the working hypothesis: "The activities of P. scaber are a means of losing by transpiration the excess of water which has accumulated in the bodies of the animals during a stay inside a very moist shelter". An experiment with artificial tree-shelters under natural conditions has yielded new data which confirm the hypothesis. With experiments in the laboratory it could be shown that specimens of P. scaber take up water from saturated or nearly-saturated air and that this results in an increasing preference for lower air humidities (for becoming active) and vice versa. Water-uptake occurs through the cuticle and the amount of water-uptake increases with temperature up to about 15-20° C. It is apparent that in saturated or nearly-saturated air, "normal" excretion cannot compensate for water-uptake, so that water is accumulated in the body. The activity patterns of P. scaber must be considered an active regulation of the water-balance, by which the animals move at intervals from a habitat with a very high air humidity (shelter) to a habitat with a lower air humidity (locality for adequate transpiration) and vice versa. It could be shown that at high humidities the animals have a greater tendency to move upwards. They then reach places where the evaporative power of the air is greater and transpiration of water is thus increased. The results from experiments in the laboratory on the effect of humidity or temperature on the amount of activity agree with corresponding findings under natural conditions. A number of unexpected results of these experiments demonstrate that the form and amount in which the activity patterns arise depend on the structure of the shelters and of the habitat in which the activities occur. Habitat-selection in P. scaber depends on the mean trend of temperature and humidity of the air during a number of years and on the presence and accessibility of suitable shelters and of localities where adequate transpiration is possible.