Insight into processes involved in child bilingualism is provided by this account of the "primary language acquisition" (in the sense of Lamendella, 1977) of two languages, English and Hebrew, by a 4 1/2 year old subject. The child's re-entry into her first language, Hebrew, after a year spent in an all-English environment, is traced. Of central concern to the discussion are the transitions between Stage I (understanding Hebrew and English but speaking only Hebrew), Stage II (all-English) and her emergence as a full bilingual after her return to the United States. This development is traced in terms of processes such as language-mixing and code-switching, with specific attention to the developmental errors in the subject's Hebrew. These errors are accompanied by increasing evidence of systematic transfer of syntactic patterns from Hebrew into English usage, but not vice versa. The overall pattern of development is analysed in terms of factors of personality and social situation as well as of the cognitive and linguistic tasks faced by the child in attaining bilingual competence in her two languages. (Author/PMJ)