Within the same learning context, learners' outcomes in terms of oral fluency vary greatly. This study tracked the relative contributions that first language (L1) and initial second language (L2) fluency skill and working memory (WM) made to L2 fluency development. We assessed the performance of French-speaking Grade 6 learners' (n = 47, mean age: 11) in a 10-month intensive English program in Quebec, Canada using a picture-cue monologic task based on The Suitcase Story and a semi-structured interview based on the Oral Proficiency Interview (OPI). Working memory was assessed using a backward digit span task and phonological memory (PM) via non-word repetition and serial non-word recognition tasks. Overall, results suggest that L1 fluency, WM and PM played only a minor role in learners' L2 fluency outcomes, whereas learners' pre-program levels of L2 fluency constituted an important predictor of L2 fluency development regardless of the speech task used to index fluency.