The experiments reported employed nonspeech analogs of speech stimuli to examine the perceptual interaction between first-formant onset frequency and voice-onset time, acoustic cues to the voicing distinction in English initial stop consonants. The nonspeech stimuli comprised two pure tones varying in relative onset time, and listeners were asked to judge the simultaneity of tone onsets. These judgments were affected by the frequency of the lower tone in a manner that parallels the influence of first-formant onset frequency on voicing judgments. This effect was shown to occur regardless of prior learning and to be systematic over a wide range of lower tone frequencies including frequencies beyond the range of possible first-formant frequencies of speech, suggesting that the effect in speech is not attributable to (tacit) knowledge of production constraints, as some current theories suggest.