This study considers the role of instruction, second language (L2) input, first language (L1)transfer, and Universal Grammar (UG) in the development of L2 morphosyntactic knowledge.Specifically, it investigates the acquisition of the Spanish morpheme se byEnglish-speaking adult learners. Participants included 91 university students and 30 Spanishnative-speaker controls. Learners received form-focused, communicative instruction onse for one week and were tested before, immediately following, and 24 days after thetreatment period. Assessment consisted of a grammaticality judgment task and two productiontasks using se in a variety of verb classes. The results showed that se hadbeen added to many learners' grammars, but also that L1-derived forms andovergeneralization errors had not been completely preempted. These findings are taken asevidence that the development of L2 grammars is affected by a number of independent, yetcooperating, knowledge sources, which thus supports a modular account of L2 acquisition.