This study investigates contextual features surrounding the use of a first language (L1) in a Japanese university English-as-a-foreign-language (EFL) course during peer interaction in the extended preparation phase leading up to two oral presentation tasks (OP1 and OP3), performed seven months apart. Interaction data were analysed in terms of the amount of L1 production, the distribution of L1 use within and across tasks and dyads, and the focus of learner talk. Contextual influences on L1 use were also investigated. The amount of L1 use increased from OP1 to OP3 and the overall proportion of L1 talk was higher than that found in previous studies. Within dyads, learners generally used less L1 over time. This was attributed to the shifting focus of talk from procedural to content-creation activity. Some learners were consistently high or low users of L1, while others varied. Variability was attributed to differences in second language (L2) proficiency, levels of engagement with the task and/or interlocutor, and the negotiation of task control and pedagogic roles within a dyad. Finally, it was found that the language chosen for the initial utterance of an exchange may influence that of following utterances. The results support the contention that L1 use emerges naturally in classroom discourse and that attempts to influence it should involve raising awareness of contextual conditions surrounding its emergence. (Contains 6 tables and 7 notes.)