Most of the population of Japan remains ethnically Japanese. For this reason, bilingualism has never been a subject of significant concern in Japan. With the recent influx of new groups of non-Japanese speakers into Japan, the society is becoming more multilingual. This book introduces the way languages are used in Japanese interlingual families, and examines what factors influence their language choice, with the aim of arriving at a predictive model of language use. Data were gathered by survey and follow-up interviews drawn from a small number of families. The book examines the "one-parent--one language" principle in relation to children's use of the minority language and suggests the alternative view of maximal engagement with the minority language. A taxonomy is proposed of interlingual family types, and a typological model of language use is presented with the intention of providing researchers with a unifying framework for the study of bilingualism. Chapter titles include the following: "Studies of Bilingualism in Interlingual Families"; "Taxonomy of Interlingual Family Types"; "Objectives and Methods of the Present Study"; "Findings"; and "Conclusions." There are also two appendices, "Language Use in Bilingual Families Questionnaire" (in English and Japanese) and a collection of 26 tables of data. A subject index is also included. (Contains 117 references.) (KFT)