A measure was obtained of the amounts of variability in fundamental voice frequency Fo in simple sentences produced by several adult male [human] speakers. A comparison was made of intra and interspeaker variability, and a detailed analysis was performed to determine the nature of this variability. Great variability was found between speakers, but the intraspeaker variability was essentially as great as the interspeaker. Even for a single speaker''s utterances, considerable noise is present which can obscure the prosodic signals. The Fo variability involved seemingly random interleavings that affected the relative Fo values within an utterance and simple shifts in average Fo that affect the absolute Fo of the contour as a whole.