Vocal tract length of 23 rhesus macaques (Macaca mulatta) was measured using radiographs and computer graphic techniques. LPC analysis of tape-recorded threat vocalizations was used to determine vocal tract resonance frequencies (``formants'') for the same animals. A new acoustic variable is proposed, ``formant dispersion,'' which theoretically depends upon vocal tract length. Formant dispersion is the averaged difference between successive formant frequencies, and was closely tied to both vocal tract length and body size. Despite the common claim that voice fundamental frequency (F0) provides an acoustic indication of body size, repeated investigations have failed to support such a relationship in many vertebrate species including humans. Formant dispersion, unlike voice pitch, is proposed to be a reliable predictor of body size in macaques, and probably many other species.