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This paper aims to identify factors that explain the level of written French proficiency in immigrants of working age. We bear in mind that little research has been conducted on this subject in France. The intention of this article is the estimation of a stochastic production frontier model to explain scores in written French of immigrants (considered as producers of their own skills) and their productive performances. The main results show that mother tongue, age at immigration, mother’s education, and social life are elements that influence the effectiveness of this production. These play a crucial role in the capacity of immigrants to effectively use their time in France, as well as their school education. These are considered as the main resources at the disposal of immigrants to produce competence in the language of the host country. JEL classification : I21, J15, J24.