This study describes rules of code-switching (choice between languages) in four six-year-old Spanish/English bilinguals in a school setting, to show that bilingualism is a sociolinguistic competency. Data included 306 episodes of situational code-switches and 64 episodes of conversational code-switches. The children consistently applied the situational rule that they speak the language their listener knew best, which contrasted with the tendency of adult bilinguals to highlight their ethnic identity. The results of the study challenge the opinion that bilingual children are "deficient" because they speak neither of their languages well; the four children observed spoke two languages fluently and controlled a range of styles within each language that demonstrated their sociolinguistic competencies. Suggestions are made for a school curriculum that promotes language maintenance through systematic interaction with monolingual speakers of both languages. (Author/RL)