The mechanisms of beat coordination in the heart of the oyster Crassostrea gigas were analyzed by examining phase-response characteristics of both the auricle and ventricle to brief stretches. Brief stretching of an isolated auricle or ventricle produced phase delay or phase advance in the following beats, in accordance with the stimulation phase. Therefore, the phase-response curve (PRC) was always biphasic and the phase shift was larger with a larger stretch. As predicted from the PRC, the beat frequency of an isolated auricle or ventricle was entrained by repeated brief stretches when the stretch frequency was in a limited frequency range (range of entrainment) around the free-running beat frequency. The range of entrainment was wider with a larger stretch. When the stretch frequency was outside the range, the beat frequency was not entrained. Instead, it changed cyclically as a process of repeated discrete phase shifts. These results suggest that the beat coordination of the auricle and ventricle is achieved by reciprocal stretching between them.