Thresholds for formant-frequency discrimination were obtained for ten synthetic English vowels patterned after a female talker. To estimate the resolution of the auditory system for these stimuli, thresholds were measured using well-trained subjects under minimal-stimulus- uncertainty procedures. Thresholds were estimated for both increments and decrements in formant frequency for the first and second formants. Reliable measurements of threshold were obtained for most formants tested, the exceptions occurring when a harmonic of the fundamental was aligned with the center frequency of the test formant. In these cases, unusually high thresholds were obtained from some subjects and asymmetrical thresholds were measured for increments versus decrements in formant frequency. Excluding those cases, thresholds for formant frequency, ΔF, are best described as a piecewise-linear function of frequency which is constant at about 14 Hz in the F1 frequency region (<800 Hz), and increases linearly in the F2 region. In the F2 region, the resolution for formant frequency is approximately 1.5%. The present thresholds are similar to previous estimates in the F1 region, but about a factor of three lower than those in the F2 region. Comparisons of these results to those for pure tones and for complex, nonspeech stimuli are discussed.