Recent longitudinal studies of simultaneously bilingual children in French and German (Meisel, J., 1990. Two First Languages. Dordrecht, Foris; Meisel, J., 1994. Bilingual First Language Acquisition. French and German Grammatical Development. Benjamins, Amsterdam) and French and English (Paradis, J. and Genesee, F., 1996. Syntactic acquisition in bilingual children, autonomous or independent? Studies in Second Language Acquisition 18, 1–25; Paradis, J. and Genesee, F. 1997. On continuity and the emergence of functional categories in bilingual first language acquisition. Language Acquisition 6, 91–124) have stressed the independent development of the two language systems. This paper presents longitudinal data from simultaneously bilingual German–English children with respect to the development of negation and syntactically related modal particles. The analysis provides evidence for language separation as well as cross-linguistic influences. The relative order of verbs and sentential modifiers appears not to be a principled syntactic operation, but to be accessed through attention to the relative order of lexical categories. These are noticed cross-linguistically, making cross-linguistic structures a logical possibility. Findings from bilingual populations are argued to have implications for the conceptualization of monolingual development as well.