This study addresses the role of output in second language (L2) acquisition by comparing processing instruction (PI) to communicative output (CO) tasks. Participants included 80 English-speaking adults from six university course sections of beginning L2 Spanish, with two assigned to each treatment (PI = 27; CO = 28) and two others comprising a control group (n = 25). Instruction lasted 7 days and targeted the anticausative clitic se. One lesson was videotaped and transcribed in each treatment group. Results on grammaticality judgment and guided production tasks administered before, immediately after, and 24 days following instruction indicated similar grammaticality judgment improvements, but more frequent uses of se by CO learners on guided production. Transcript data suggest a role for output in acquisition involving attention to, and metalinguistic analyses of, L2 structure.