Simulation of natural locomotion in virtual environments requires an understanding of self-motion perception derived from both gait and sensory parameters. The walk-run transition (WRT) is a clearly-defined physical gait descriptor that emerges largely from a user's assessment of their own speed. The WRT depends not only on the biomechanics, kinetics and energetics of gait, but also on perceptual and cognitive factors, such as optic flow [Mohler et al. 2004; 2007], attention [Pelah et al 2006] and step frequency [Durgin et al 2007]. To further investigate gait transitions and the mechanisms for perception of self motion speed, 15 subjects were required to advance at prescribed step frequencies with increasing speeds on a treadmill, with or without optic flow, in a large-screen virtual environment. WRT speed reached its highest value at approximately 145 steps/min (0.61 Fr) for all conditions, decreasing at higher step frequencies. Above this frequency, optic flow reduced WRT speeds as step frequency increased, suggesting an enhancement of visuomotor interactions in locomotion simulators due to step frequency being forced beyond the natural step frequency range.