1. Experiments were done on the reactions of adult B . geniculosa in relation to various environmental factors. 2. In a range of different sand fractions in saturated air, the mites always moved to the largest sand particles. The finer the sand, the more intensely it was avoided. 3· In a relative humidity gradient and in a gradient of different sand fractions, they chose fine sand over a high humidity in preference to coarse sand over a low humidity. 4· The mites chose a dry, smooth substratum in preference to a moist, smooth one; this behaviour was reversed when they became sufficiently desiccated. They chose coarse, moist sand in preference to fine, dry sand. 5. There was no reaction to dorsal monochromatic light. 6. The mites responded to horizontal monochromatic light, depenciing upon the illuminance. They were photonegative between s,ooo lux-r,ooo lux, and photopositive between 300 lux-100 lux; their reactions varied from 1,000 lux-300 lux. They appeared indifferent to very dim light (below 100 lux). 7· There was positive response to gravity.