This study addresses the role of perception in interlimb coordination. In a set of four experiments, observers judged the relative phasing of two balls oscillating on a computer display. Balls were simulated to move in the frontoparallel plane (Experiments 1 and 2) or in depth (Experiments 3 and 4). Under each viewing condition, participants observed the same displays, but were to judge either mean relative phase (Experiments 1 and 3) or phase variability (Experiments 2 and 4). Added phase variability interfered with the mean relative phase judgments, in particular in the in-phase conditions. Judgments of phase variability were better explained by mean relative phase. Furthermore, the stability of the judgments followed an asymmetric inverted U-shaped relation with mean relative phase, as described by the Haken-Kelso-Bunz model. We conclude that relative phase is a perceptual variable.