The present article reports on the findings of a case study of bilingual first language acquisition in Catalan and English. It first presents a general overview of a child's syntactic development from the age of 1;3 to 4;2 and then focuses on the question of subject realization in the two contrasting languages he is acquiring simultaneously. In this case, Catalan is a null subject language in opposition to the overt subject properties of English. Such data allow us to provide evidence on a key issue in bilingual acquisition research: the question of language separation in the early stages of acquisition. The data available suggest the absence of any major influence of one language on the other. In other words, our subject seems to be acquiring word order patterns which are different in the two adult systems in a language-dependent manner from the beginning of his production in both languages.