A naturalistic study looked at the acquisition of English by two Japanese boys, aged 4 and 8 years, during a 2.5-year stay in the United States. Data were collected through observation and transcription of spontaneous speech in daily life. Analysis included a variety of features of language use and of the acquisition process, including attitudes toward English, stages in utterance development, code-switching and mixing, developmental commonalities (affective language, formulaic utterances), and suprasegmental features and patterns. Findings are related to theories of cognitive development, models of bilingualism, and a hypothesis of developmental interdependence in first and second language proficiency. It is concluded that two factors, age and first language proficiency, are crucial to whether a child develops a coordinate or compound bilingual system. Contains 18 references. (MSE)