Female tsetse flies (Glossina morsitans Westw.) were more active in an experimental chamber in the laboratory: (a) at high than at low temperatures, (b) in high than in low humidities, and except at high temperatures, (c) when hungry than when replete. Flies were also more active in the chamber when a small area of black surface (1·6%) was present than when a larger black surface (12·8% or 50%) was available. Both black and white surfaces were provided in all experiments, and over the temperature range 25–40°C flies settled on black more frequently than would be expected by chance. This preference for a black resting surface was most marked when the flies were most active: (a) above 35°C in all experiments, (b) at high humidities with teneral flies, and (c) when the flies had not fed. Flies settling on black usually rested longer than those settling on a white surface.