1. The lethal effects of temperature extremes with different species of oribatid mites were studied. 2. Each species has a characteristic thermal death point. 3· The TDP also depends on the relative humidity and on the length of exposure. 4· With B. geniculosa, death after IS minutes or after I hour exposures is caused mainly by lethal heat at all relative humidities. After rz hours it is caused mainly by excessive water-loss at low humidities and mainly by lethal heat at high humidities. 5. There is a broad relationship between the TDP of the mites and their different habitats. 6. The TDP of B. geniculosa remains unchanged when previously kept at 0.7°C-25°c; they are thus not acclimatized to these temperatures. 7· The cold death temperature of varions species of mites after r hour varies from -3°C to -12°C. 8. The cold death temperature of B. geniculosa was not changed when previously kept at 0.7°C to 25°C. 9. There is no correlation between the cold death temperature and the natural habitat of the mites.