This paper describes the use of a treadmill-based virtual environment (VE) to investigate the influence of visual motion on locomotion. First, we demonstrate that a computer-controlled treadmill coupled with a wide field of view computer graphics display can be used to study interactions between perception and action. Previous work has demonstrated that humans calibrate their visually-directed actions to changing circumstances in their environment. Using a treadmill VE, we show that calibration of action is reflected in the real world as a result of manipulating the relation between the speed of visual flow, presented using computer graphics, and the speed of walking on a treadmill. Second, we extend the methodology in our treadmill VE to investigate an open question involving human gait transitions and show that the speed of visual motion influences the speed at which the gait transition occurs. These results demonstrate both the effectiveness of treadmill-based VEs in simulating the perceptual-motor effects of walking through the real world and the value of such systems in addressing basic perceptual questions that would otherwise be difficult to explore.