This study builds upon previous research investigating the construct validity of phraseological complexity as an index of L2 development and proficiency. Whereas previous studies have focused on cross-sectional comparisons of written productions across proficiency levels, the current study compares the "longitudinal" development of phraseological complexity in written "and oral" productions elicited over a 21-month period from learners of French. We also improve upon the state of the art by including L1 data to benchmark learner levels of phraseological complexity. Phraseological complexity, operationalized as the diversity (no. types) and sophistication (PMI) of adjectival modifiers (adjective + noun) and direct objects (verb + noun), was generally higher in learner writing as compared to speaking. Over the study period, the sophistication of phraseological units increased slightly but developmental patterns were found to differ between tasks, highlighting the importance of considering task characteristics when measuring phraseological complexity.