A psychoacoustic experiment to determine the just-discriminable changes in the fundamental frequency of synthetic vowels is described. The experimental parameters investigated are several combinations of four vowels (|i, æ, ɑ, u|), three sound pressure levels (60, 70, and 80 db re 0.0002 dyne/cm2) and two fundamental frequencies (80 and 120 cps). The results indicate that the just-discriminable changes in fundamental frequency are of the order of 0.3 to 0.5 cps, and are, in general, slightly less than the frequency changes discriminable a pure tone of the same frequency and sound pressure level. Application of the the results to speech compression is suggested.