The transition speed between walking and running was determined in 18 healthy males (23-38 years) of various statures (range of leg length 0.86-1.09 m) during acceleration and deceleration of different magnitudes (0.05-0.11 ms-2) on a motor-driven treadmill. The speed of the treadmill belt and the duration of the stance phases of each foot were recorded. A transition step was identified and the speed of transition was taken as the average speed value of the stance phase of that step. The overall mean value for the transition speed was 1.88 m s-1 (range 1.30-2.55). Deceleration resulted in a somewhat lower speed of transition than acceleration. There was a tendency towards increasing values for transition speed with increasing leg length. This could partly be explained by differences in natural frequency. The reasons for the switch between walking and running at a speed which is not extreme for either mode of progression are unclear. The subjective feeling that a transition will lead to a more comfortable situation might be based on previous experience combined with information from peripheral receptors and activity in central networks controlling locomotion.