Frequency modulation of speech is an effect produced by periodically modulating the speech frequencies during transmission. This effect is detectable by the wavering or buzzing quality that it imparts to the speech as received. The detection limits of frequency modulation of speech were determined using the Békésy tracking technique. Modulation rates from 10 to 200 cps were used on speech produced by both male and female talkers. Detection limits, expressed as a function of signal to sideband ratio, were found to vary as a function of sex of the talker, modulation rate, and received‐speech level. Detectability is greater with female than with male talkers, and peaks at a higher modulation rate for the female talkers than for the male talkers.