In this case study we investigate how early a developing bilingual exposed simultaneously to English and Spanish can make appropriate language choices. We propose two methodological requirements for studies of this kind: (1) detailed records of the development of the child's lexicon; and (2) data collection in more than one language context. Our own study relies on detailed records of the child's cumulative vocabulary from the first word at ten months, and on weekly audiovideo recordings in both English and Spanish contexts from age 1;3. Analysis of the data shows a strong tendency for the language of the child's utterances to match that of the context at ages 1;7–1;8.