Purpose: The capacity of 6-month-old infants to discriminate a voicing contrast (/aba/--/apa/) on the basis of "amplitude modulation (AM) cues" and "frequency modulation (FM) cues" was evaluated. Method: Several vocoded speech conditions were designed to either degrade FM cues in 4 or 32 bands or degrade AM in 32 bands. Infants were familiarized to the vocoded stimuli for a period of either 1 or 2 min. Vocoded speech discrimination was assessed using the head-turn preference procedure. Results: Infants discriminated /aba/ from /apa/ in each condition; however, familiarization time was found to influence strongly infants' responses (i.e., their preference for novel vs. familiar stimuli). Conclusions: Six-month-old infants do not require FM cues and can use the slowest (< 16 Hz) AM cues to discriminate voicing. Moreover, 6-month-old infants can use AM cues extracted from only 4 broad-frequency bands to discriminate voicing.