Recent research on American English /r/ and /l/ has suggested that adult Japanese speakers are more sensitive to changes in F2 frequency than to changes in F3 frequency. In contrast, adult American English speakers seem more sensitive to changes in F3 frequency. To examine the developmental origins of these cue weightings, 7.5-month-old infants raised in monolingual English homes were tested. Following the head-turn conditioning procedure, infants were trained to turn toward a visual reinforcer whenever a continuously repeated background stimulus was changed to a test stimulus. In an initial conditioning phase, infants were trained to discriminate two synthetic /ra/ and /la/ stimuli that differed in both F2 and F3. In the following test phase, infants discriminated these two stimuli along with stimuli that differed from the background along only one of these dimensions. The results demonstrated that infants were more sensitive to F2 than to F3, suggesting that their cue weightings may better match those of Japanese adults.