The traditional interpretation of language mixing by bilingual infants is that such language use is a reflection of a single undifferentiated language system (Volterra and Taeschner 1978; Redlinger and Park 1980; Vihman 1985). Recent studies claim, however, that young children's mixing can be attributed to the mixed utterances of their parents and their interactional strategies towards the child's mixing (Goodz 1989; Lanza 1992, 1993). Longitudinal observation of natural interaction between an English/Japanese bilingual infant (from the age of 1;10 to 2;2) and each of his parents was conducted to confirm the hypothesis above. The results provide additional supportive evidence for the current view on language mixing, suggesting that language mixing indicates the development of the pragmatic competence to differentiate between the two languages according to the discourse context. Based on the results, I offer further discussion of the issues of language mixing and language differentiation in bilingual development.