This study examines the roles of proficiency and working memory (WM) capacity in second-/foreign-language (L2) learners' processing of agreement morphology. It investigates the processing of grammatical and ungrammatical short- and long-distance number agreement dependencies by native English speakers at two proficiencies in French, and the relationship between their proficiency and WM capacity in French and their sensitivity to agreement violations. Native English speakers at mid- and high proficiencies in French and native French speakers completed an acceptability judgment task, a self-paced reading task, and a WM task in French, and the English speakers also completed a WM task in English. The results showed that whereas all participants performed at ceiling on the acceptability judgment tasks, only the high-level L2 learners and native speakers showed some sensitivity to number agreement violations. For L2 learners, this sensitivity did not vary as a function of the length of the agreement dependency. The results also indicated that L2 learners tended to be more sensitive to agreement violations as their WM memory capacity in French increased. The implications of these results for theories of L2 morphological processing are discussed.