The present study investigated Japanese students' EFL vocabulary development through e-mail interactions with a native English speaker (NS), with primary focus on students' imitation of new words. According to sociocultural theory, learners can internalize new linguistic knowledge by imitating an expert's expressions to create his/her own utterances. This study, therefore, specifically examined whether (1) students could imitate the new vocabulary items provided by the NS tutor, and (2) they could retain these items at the end of the project. An examination of the e-mail log showed that students did imitate and use some of the lexis provided by the NS. The results of the post-test also revealed that some of the imitated words were subsequently retained. However, there were some words that had been learned without imitation. The questionnaire survey and the students' interview comments indicated that students memorized a considerable number of new words provided by the NS by repeatedly reading them in e-mail text, as well as in other learning contexts, such as regular classes and independent study, through noticing, retrieving, and generating the meaning/form of each word (Nation, 2001). The study concluded that vocabulary learning via e-mail takes place not only by a single process such as imitation, but also by a combination of various processes functioning in an integrated manner.