Variations in the acoustic manifestations of the rounding and height contrasts in French vowels are investigated in different prosodic positions. Four speakers produced sentences containing the vowels /i, e, a, y, ø/ at the beginning of different prosodic constituents: Intonational Phrase, Accentual Phrase and Word. Acoustic cues for the rounding contrast (F2, F3, F3-F2) and for the vowel height contrast (F1) are found to be enhanced in Intonational Phrase initial positions.