This paper explores the longitudinal development of the phrasicon (i.e. phraseological lexicon) of French-speaking learners of Dutch in two different educational settings: Content and Language Integrated Learning (CLIL) and traditional foreign language learning contexts (non-CLIL). We followed 195 pupils and analyzed a corpus of 390 written texts gathered at two data collection points: at the beginning of their fifth year of secondary school education (Grade 11) and at the end of their sixth year (Grade 12). We examined both frequency and accuracy of the phrasicon. Whilst SLA research often supports the fact that learners at a higher proficiency level have a larger phrasicon and that CLIL learners' overall language competence is higher than their non-CLIL peers, our results show no significant increase in the frequency of use of phraseological units in CLIL; a significant decrease for two categories (referential phrasemes and lexical collocations) is even noticed. non-CLIL learners display no significant change in the use of various types of phrasemes. Regarding accuracy, both CLIL and non-CLIL learners' phrasicon become more accurate (fewer errors) over time in more than 50% of the categories. Here again, no statistically significant improvement was noted among the groups between the two data collection points.