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 by English-speaking adult learners. Participants included 91 university students and 30 Spanish native-speaker controls. Learners received form-focused, communicative instruction on se for one week and were tested before, immediately following, and 24 days after the treatment period. Assessment consisted of a grammaticality judgment task and two production tasks using se in a variety of verb classes. The results showed that se had been added to many learners' grammars, but also that L1-derived forms and overgeneralization errors had not been completely preempted. These findings are taken as evidence that the development of L2 grammars is affected by a number of independent, yet cooperating, knowledge sources, which thus supports a modular account of L2 acquisition.