This study examines the processing of agreement in continuous speech by English-speaking late learners of French as a second/foreign language (L2). It aims to tease apart three theories that attempt to explain why the processing of inflectional morphology is difficult for late L2 learners. Native English speakers who began learning French towards the age of twelve and native French speakers completed an auditory acceptability judgment task in French in which subject-verb agreement dependencies were either short (adjacent positions) or long (non-adjacent positions). Participants also completed a series of tests that could serve as individual variability measures: a French proficiency test (cloze test), two working memory tests (one in each of French and English), a word-familiarity questionnaire targeting the verbs used in the acceptability judgment task, and two language aptitude tests not specific to French or English (phonological memory and grammatical deduction). The results of the acceptability judgment task show lower sensitivity to agreement in L2 learners than in native speakers, and an effect of distance between the subject and the verb only in L2 learners. Regression analyses reveal that proficiency in French and phonological memory explain some of the variance in the acceptability judgments. We discuss the implications of these findings in relation to the three theories that attempt to explain the lack of sensitivity to agreement in late L2 learners.