This paper examines how context is configured in ESL students' language learning practices through computer-mediated communication (CMC). Specifically, I focus on how a group of ESL students jointly constructed the context of their CMC activities through interactional patterns and norms, and how configured affordances within the CMC environment mediated their learning experiences. After a brief review of relevant studies of CMC in the literature, I discuss ecological perspectives of language learning as a core construct of this study, to explain contextual fluidity in relation to learners' agency in their learning. Next, I present an ethnographic study of how members of an ESL class constructed a community of social practices through synchronous CMC. The findings indicate that (a) the constructed interactional patterns and norms of the students' CMC activities represented group dynamics among the participants, (b) the participants' roles in joint construction of the activities reflected their language socialization experiences, and (c) the activities provided a way for spousal participants to assume academic identities, while becoming a social space for academic gatherings. This study highlights the fluidity of CMC language learning contexts; fluid contexts entail learners' agency in dialogic engagements with the contextual elements of the learning environment as language socialization processes. (Contains 1 table, 1 figure and 11 notes.)