Recent studies have shown that even highly-proficient Spanish-English bilinguals, who acquired their second language (L2) in childhood and have little or no foreign accent in English, may require more acoustic information than monolinguals in order to identify English vowels and may have more difficulty than monolinguals in understanding speech in noise or reverberation (Mayo, Florentine, and Buus, 1997; Febo, 2003). One explanation that may account for this difference is that bilingual listeners use acoustic cues for vowel identification differently from monolinguals (Flege, 1995).In this study, we investigated this hypothesis by comparing bilingual listeners use of acoustic cues to vowel identification to that of monolinguals for six American English vowels presented under listening conditions created to manipulate the acoustic cues of vowel formant dynamics and duration. Three listener groups were tested: monolinguals, highly proficient bilinguals, and less proficient bilinguals. Stimulus creation included recording of six target vowels (/i, I, eI, E, ae, A/) in /bVd/ context, spoken in a carrier phrase by four American monolinguals (two females, two males). Six listening conditions were created: 1) whole word, 2) isolated vowel, 3) resynthesized with no change, 4) resynthesized with neutralized duration, 5) resynthesized with flattened formants, and 6) resynthesized with flattened formats and neutralized duration. The resynthesized stimuli were created using high-fidelity synthesis procedures (Straight; Kawahara, Masuda-Katsuse, and Cheveigne 1998) and digital manipulation. A six-alternative forced choice listening task was used. The main experiment was composed of 240 isolated vowel trials and 48 whole word trials.