Discussions of music performance often stress diversity and artistic freedom, yet there is general agreement that interpretation is not arbitrary and that there are standards that performances can be judged by. However, there have been few objective demonstrations of any extant constraints on music performance and judgment, particularly at the level of expressive microstructure. This study illustrates such a constraint in one specific case: the expressive timing of a melodic gesture that occurs repeatedly in Robert Schumann's famous piano piece, "Traumerei." Tone onset timing measurements in 28 recorded performances by famous pianists suggest that the most common " temporal shape" of this (nominally isochronous) musical gesture is parabolic and that individual variations can be described largely by varying a single degree of freedom of the parabolic timing function. The aesthetic validity of this apparent constraint on local performance timing was investigated in a perceptual experiment. Listeners judged a variety of timing patterns (original parabolic, shifted parabolic, and nonparabolic) imposed on the same melodic gesture, produced on an electronic piano under control of a Musical Instrument Digital Interface (MIDI). The original parabolic patterns received the highest ratings from musically trained listeners. (Musically untrained listeners were unable to give consistent judgments.) The results support the hypothesis that there are classes of optimal temporal shapes for melodic gestures in music performance and that musically acculturated listeners know and expect these shapes. Being classes of shapes, they represent flexible constraints within which artistic freedom and individual preference can manifest themselves.