How online interactants advance their talk in written interactions has been extensively investigated in different areas, including language learning. Applying a conversation analysis technique, this study examined how second language (L2) learners managed topics in L2 task-based text-chat interactions and the effects of L2 proficiency on their interactional practices. Participants were 53 dyads, comprising 106 Japanese learners of English across three proficiency levels. The findings demonstrated that only highproficiency learners jointly constructed differences in topicality between on- and off-task interactions, by implementing resources available in the textual communication medium. Mid- and low-proficiency learners tended to recycle similar formats to create new sequences. In addition, low-proficiency learners never showed orientation to topic transition in an observable way; mid-proficiency learners did show an indication of such an orientation to topic transition, although they failed to achieve it interactionally. The findings were partially verified by quantifying the question format "How about you?," which was predominantly used by low- and mid-proficiency learners. Based on the findings, this study proposes a hypothetical developmental pathway and pedagogical implications for teaching and assessment of interactional competencies in L2 written interactions.