This study investigates an instantiation of a 3D virtual world-based Japanese learning curriculum within the context of a Japanese as a foreign language (JFL) classroom. Through a mixed-method case study approach, participants' natural acquisition of Japanese in a 3D virtual environment was examined. In the present study, four sources of data from 11 university-level JFL students (n=11) were collected and analyzed to evaluate the learning outcomes from a new instructional approach called computer-assisted learning of communication (CALC). Based on both interpretative and statistical analyses of data, the major finding of the present study was that the participants, when immersed in the 3D virtual world of Tokyo, acquired contextualized communicative competence. More specifically, quantitative analyses revealed statistically significant improvement in the participants' acquisition of incidentally encountered vocabulary, in particular, kanji pronunciation and vocabulary interpretation. Qualitative analyses revealed participants' acquisition of various communicative competencies specific to the context, including persuasive talk, awareness of audience, and collaborative communication.