This study investigated whether the role of working memory capacity varies over the course of second language (L2) morphosyntactic development. Eighty-seven beginning, intermediate, and advanced university L2 Spanish learners completed two nonverbal tasks measuring executive function (EF) and phonological working memory (PWM) in their native language (English) and two tasks measuring knowledge of ten grammatical structures in Spanish at three points during and after a semester of instruction. Robust relationships between both working memory components, especially PWM, and L2 performance, emerged only for lower level learners, particularly at the start of instruction and 3.5 months later. Findings demonstrate that the facilitative effects of cognitive ability appear to lessen with increasing L2 profciency and empirically support a developmental perspective of L2 learning.