This research investigated the role of child-directed speech in the acquisition of vowel systems in a cross-linguistic perspective. In order to determine if vocalic systems are extended in child-directed speech and if this extension varies cross-linguistically, child-directed speech was compared to adult-directed speech in three different languages, French, English and Japanese. The same short story was successively read by mothers to their infant and to an adult (5 mother-infant dyads per language). The acoustic analyses reveal a downward shift of the vowel triangle on the high-low dimension of vowel space (F1). In the three languages, mothers tend to produce more opened vowels in CDS than in ADS.