This paper reports the preliminary results of a study examining the role of structural overlap, language exposure, and language use on cross-linguistic influence (CLI) in bilingual first language acquisition. We focus on the longitudinal development of determiners in a corpus of two French-English children between the ages of ; and ;. The results display bi-directional CLI in the rate of development, i.e., accelerated development in English and a minor delay in French. Unidirectional CLI from English to French was instead observed in the significantly higher rate of ungrammatical determiner omissions in plural and generic contexts than in singular specific contexts in French. These findings suggest that other language-internal mechanisms may be at play. They also lend support to the role of expressive abilities on the magnitude of this phenomenon. The oretical bac kground Although bilingual children's linguistic development largely follows the path of monolinguals (De Houwer, ), there is ample evidence that bilinguals' linguistic behaviour may occasionally differ from a quantitative and/or qualitative standpoint from that of monolingual children. This