ABSTRACT The transpiration of three species of Isopoda has been investigated ; Armadillidium shows the highest resistance to desiccation, Oniscus the least, and Porcellio is intermediate. The permeability of isopod cuticle is limited by lipoids which impregnate the endocuticle. If the temperature is raised above the lipoid melting-point a marked increase in permeability results. Isopods show a characteristic decline in transpiration rate during exposure to desiccating atmospheres. The high initial rate of evaporation is due mainly to loss of water from layers of the cuticle external to the lipoid barrier. There is a progressive decrease in the permeability of the integument during desiccation. This is brought about by a shrinkage of the cuticle, which leads to a closer packing of lipoid molecules. The mechanism of this structural alteration is based on an interaction between endocuticular proteins and the ionic constituents of body fluids; in the course of desiccation the blood concentration increases and there is a corresponding decrease in the dispersion of endocuticular elements.