Behavioral and acoustic reflex thresholds were determined for five normal-hearing subjects in response to carrier signals of 500 and 2000 Hz which were unmodulated or modulated sinusoidally at rates of 2, 20, and 200 times per second with frequency deviations (delta f) of 30, 100, and 300 Hz. Behavioral (absolute and masked) thresholds were determined using an adaptive two-alternative forced-choice procedure. Acoustic reflex thresholds were determined by visual inspection of stored reflex waveforms. Frequency modulation was not found to exert a systematic effect at absolute threshold. Frequency modulation did affect threshold estimates systematically, but differentially, at masked threshold and acoustic reflex threshold. Increasing the frequency deviation of the modulation was associated with an increase in masked threshold and with a decrease in acoustic reflex threshold at both test frequencies. The findings are discussed in terms of critical band phenomena.