This study explores interaction between the vowel systems of German-Spanish bilingual children. As German is characterized by a larger vowel inventory system than Spanish and an opposition between long and short vowels, bilingual children were delayed in the acquisition of the German vowel length compared to monolingual children, whilst displaying a similar performance as monolingual children in the production of Spanish vowels. The relevance of these results for bilingual acquisition and acquisition of vowel length representation in children's grammars is commented on.