The effect of tonal structure on rhythm in piano performance was examined. Although music structure is viewed as having rhythmic and tonal aspects, the tonality of a score might alter the rhythm in performance. Five skilled pianists sight read short segments of music under different tonal conditions. Timing in performance was monitored, and the variability of successive intervals was used as a measure of rhythmic precision. Results indicate significant differences between tonal and atonal conditions for the rhythm in performance. Possible explanations are considered, including the past experience of the pianists, expression in performance, biomechanical transitions, and other aspects of music structure visible from the score.