This paper has as its starting point the assumption that in acquiring two languages from birth, bilingual children separate their grammars from very early on. This does not, however, exclude cross-linguistic influence – the possible influence of one language on the other. The main focus of the paper is on the acquisition of syntax in a generative framework. We argue that cross-linguistic influence can occur if (1) an interface level between two modules of grammar is involved, and (2) the two languages overlap at the surface level. We show that both conditions hold for object drop, but not for root infinitives. Root infinitives satisfy the first condition: they involve the interface between syntax and pragmatics. However, they do not satisfy the second condition. Therefore, we expect cross-linguistic influence to occur only in the domain of object drop and not in the domain of root infinitives. Comparing the development of the two phenomena in a bilingual Dutch–French and a German–Italian child to the development in monolingual children, we show that this prediction is borne out by our data. Moreover, this confirms the hypothesis that cross-linguistic influence is due to language internal factors and not to language external factors such as language dominance: the periods during which we observe influence in the domain of object drop and non-influence in the domain of root infinitives are identical.