1. Oribatid mites from various habitats were kept at different saturation deficits in constant temperatures of 5, r5 and 25° C. 2. The mites generally survived much longer at low temperature than at high. 3. As a rule, the lower the saturation deficit, the longer the length of life. However, at 25°C H . rostrolamellatus lived much longer in dry air than in moist air. Possible reasons for this are discussed. 4· There is a hyperbolic-shaped relationship between mean longevity and saturation deficit; the reciprocal of mean longevity and saturation deficit is curvilinear for most species and linear for H. ntfulus and N . nana. Reasons for this are given. 5. The longevity of H. rostrolamellahts and B. geniculosa in relation to waterloss at different saturation deficits is described and discussed. 6. The mean longevity of the different species of mites in different saturation deficits is generally correlated with the amount of moisture available in their habitat.