1.This paper describes the reactions of a common litter-inhabiting oribatid mite, Belba geniculosa Oudms., in a small temperature gradient apparatus. 2. Notwithstanding marked differences in relative humidity and evaporating gradients inside the experimental chamber in different experiments, both the adults and the juveniles (tritonymphs) invariably aggregated in a narrow temperature range of 11.0° C-15.0° C (mean 13.5° C) and 11.0° C-15.0° C (mean 13.0° C) respectively. 3· Desiccation lowered the preferred temperature of adult mites by over 2° C when compared to normal mites. 4· Acclimatization to varions temperatures had little effect on the preferred temperatures of the mites. 5· Individual mites had a slightly different preferred temperature from groups of mites. 6. Reactions to radiant heat were well-marked, the adults choosing 13° C (range 12.5° C-13.5° C) and the juveniles r.so C less (range 10.0° C-13.0° C). 7· A brief account of the orientating mechanism of B. geniculosa in the temperature gradient apparatus is given. 8. Three other species of oribatid mites chose characteristic preferred temperatures, which were similar in either dry or moist air. 9· Sorne theoretical considerations and practicallimitations in the use of closed temperature gradients are given. 10. It is concluded that B. geniculosa has a true preferred temperature, irrespective of other prevailing physical environmental conditions.