This study investigated the effect of wiki-mediated collaborative writing on the development of learners' individual writing in a second language (L2). Participants were 52 learners of English as a foreign language enrolled in two intact junior writing classes at a Taiwanese university. One class was assigned to be a wiki-collaborative writing group (n = 26), and the other an individual writing group (n = 26). Both classes participated in an individual pre-test writing, a writing task, and an individual post-test writing over a 9-week period. Students in the wiki group worked in pairs via wikis to produce an expository essay; students in the individual group produced their essays alone. Learners' written production on the pre-test and post-test was analyzed in terms of content and organization, and linguistic complexity and accuracy. Results indicated a significant effect for wiki-mediated collaborative writing on the content quality and linguistic accuracy of learners' individual writing in L2. Its effect on the organization and linguistic complexity, however, was less evident.