Although the high-water Talitridae are normally terrestrial they show little structural modification compared with aquatic amphipods. They have no obvious adaptations to limit water-loss, and their branchial method of respiration is typically aquatic. It was to be expected, therefore, that the evaporating power of the air would play a considerable part in limiting their habitats and determining their habits. The following experiments were made to try and assess the importance of this factor.