From obsevations on larvae from laboratory-reared females of Glossina morsitans orientalis Vanderplank, six stages were recognised between the time of deposition (signalled in these experiments by a simple automatic device) and completion of puparium formation, each characterised by an associated pattern of behaviour. They are termed crawling (during which there is a stron phot-negative response), circling and reversing, head retraction, barrelling (the assumption of the puparial shape), hardening, and darkening. In choice-chamber experiments, crawling larvae markedly preferred moist to dry air but showed no response to gravity. In daylight at 26°C., larvae introduced into translucent plastic tubing travelled up to 65 in. before pupating; under bettwe conditions for locomotion they would probably go further. The interval before pupation was longer the earlier in the day birth occurred and was greater in light (average 108 min.) than in darkness (average 40 min.) and least, perhaps because of the contact stimulation, in sand in darkness. When bodily contact was minimised by suspending larvae by a thread tied round or stuck to the polypneustic lobes, pupation was accelerated; this possible reflected the mechanical stimulation of the handling, for pinching the larva with forceps produced a similar acceleration.