Informed by sociocultural theory and guided especially by "collective scaffolding", this study investigated the nature of computer-mediated interaction of three groups of English as a Foreign Language students when they performed collaborative writing tasks using wikis. Nine college students from a Chinese university participated in the wiki-mediated collaborative writing project. Analyses of data from the wiki "Discussion", "Page", and "History" modules on each group tab revealed that the three small groups displayed three distinct patterns of online interaction: collectively contributing/mutually supportive, authoritative/responsive, and dominant/withdrawn. These patterns were substantiated by the roles group members assumed and members' task approaches in terms of equality and mutuality. Also, findings from semi-structured interviews suggested that the different patterns of interaction influenced the students' perceived learning experiences. The collectively contributing/mutually supportive group reported the most learning opportunities. This study not only fills a gap in current collaborative writing literature but also makes a theoretical contribution to research on computer-mediated interaction in collaborative learning. Pedagogical implications on how to conduct computer-mediated collaborative writing are also addressed. (Contains 4 tables and 8 notes.)