Stimuli consisting of two simultaneous and sinusoidally frequency-modulated pure tones were dichotically presented to four listeners. Two component tones of each stimulus were approximately an octave apart. They were both modulated at 2 Hz, and the frequency swing resulting from each modulation corresponded to one tenth of the carrier frequency. The listeners' task was to detect phase differences between the modulation waveforms of the two simultaneous tones: With an adaptive 2IFC procedure, just-noticeable values of phi, the phase angle of the modulation waveforms, were measured as a function of the interval formed by the carrier frequencies (one octave, i.e., 1200 cents, +/- 0, 25, 50, or 100 cents). When the carrier frequencies were not too high, just-noticeable values of phi often varied nonmonotonically with the interval, showing a minimum at or near 1200 cents. An additional experiment indicated that most, if not all, of these octave effects were not due to some form of beat detection. As a whole, the results reported here provide evidence for the existence of internal octave templates. Such templates might play an important role in the perceptual segregation of simultaneous harmonic signals, as well as in pitch perception.