ABSTRACT Oxygen uptake and carbon dioxide output were measured in the woodlice Ligia oceanica, Oniscus asellus, Porcellio scaber and Armadillidium vulgare, in moist and in dry air, using normal animals and animals whose pleopods had been blocked by emulsion paint. In moist air, the rate of oxygen uptake which occurred through the general integument in animals whose pleopods had been blocked was about 50 % of normal in Ligia and Oniscus, but less than this in Porcellio and Armadillidium. After exposure to moving dry air, oxygen uptake in dry air was below normal in intact animals and nearly ceased in animals with blocked pleopods. Both showed a somewhat higher uptake if the oxygen tension was raised. Carbon dioxide output in moist air was not immediately inhibited by blocking the pleopods, suggesting free diffusion through the integument. Output fell during the first hour, but the R.Q. remained above 1 in all species. Survival times of blocked animals in moist air corresponded qualitatively with rates of oxygen absorption in the four species. In dry air survival was further curtailed, but prolonged again by an increased oxygen tension. Blocked animals of all species survived for a shorter time in moist nitrogen than in moist air. Injection of reduced indigo showed that oxygen was absorbed by intact animals through the thin ventral integument and bases of the legs in Ligia, and to a lesser extent, through the ventral abdominal integument in Oniscus and Porcellio. The ecological implications of these results are discussed, particularly with reference to the relation between survival and humidity. The dehydration death point is probably affected by the level of oxygen deficit and vice versa. A water-permeable integument is of value for respiration as well as for temperature control, but restricts the habitat range of terrestrial animals.