This paper presents a longitudinal multiple case study that focused on the development of the written interlanguage of four learners over their three years of learning French as a second language (L2) in upper secondary school in Norway. The development of macro-level syntactic complexity and the development of micro-level measures of accuracy in morphosyntactic features specific to written French were examined. In order to trace development of syntactic complexity, we used two well-established measures in the SLA literature: T-unit length and the number of dependent clauses per T-unit (Wolfe-Quintero et al., 1998; Verspoor et al., 2017), whereas the morphosyntactic features representing the basis for our analysis of accuracy were gender and number in the noun phrase in French. The results illustrated that the written French of all four learners clearly became more syntactically complex over the three years of the study. As regards the development at the morphosyntactic level, progress was less straight forward in the individual learners, and the developmental tendencies varied according to the nature of the features studied.