The present study examined the extent to which first language (L1) awareness can benefit second language (L2) grammatical learning of the French "Imparfait", a crosslinguistically complex target feature. Sixty-nine English-speaking learners of L2 French received different types of explicit information (EI) about L2 or L2 + L1 form-meaning mappings. A 'core' treatment received by all learners consisted of EI about L2 with L2 comprehension-based practice of French sentences. Two further treatments examined the impact of additional (i) EI about L1 and (ii) comprehension practice of L1 sentences. Results from an online self-paced reading test, offline judgment tests (in reading and listening) and an oral sentence completion test with source of knowledge probes showed that performance immediately after the instruction and then six weeks later improved only for learners exhibiting L1 awareness. These results suggest that awareness of L1 form-meaning mappings can benefit L2 grammar learning of a crosslinguistically complex target feature.