This study investigates bilingualism among young learners in English and Japanese
immersion education in Japan through analysis of their mother tongues and language use. It involves 126 participants: elementary school students (N = 42) and their fathers (N = 42) and mothers (N = 42). The results demonstrate that while most of the parents use only Japanese between each other in their conversations, the mother tongues of the students are in some cases English and in others Japanese, while yet other students are bilingual in English and Japanese. This diversity of mother tongue among the students was found to be related to their amount of experience abroad (with length of time and location as factors), but being educated by parents who had lived abroad was found to affect students’ mother tongues a fortiori. With regard to language use, there was no great difference among the three mother tongue groups, and the students commonly had both English and Japanese input and output in multiple environments such as family milieu, school milieu, and socioinstitutional milieu. In sum, this study suggests that although the status of a language as mother tongue can be fragile in certain language environments, young learners tend to become bilingual if, in addition to receiving immersion education at school, they have parents who cooperate in passing on to their offspring languages and cultures with which they have a common affinity.