Relations between bilingual children's patterns of conversational code-switching (responding to one language with another), the balance of their dual language input, and their expressive and receptive proficiency in two languages were examined in 115 2½-year-old simultaneous Spanish-English bilinguals in the U.S. Children were more likely to code-switch in response to Spanish than English. Children's expressive vocabulary scores were higher in English than in Spanish, while their English and Spanish receptive language scores were not different. Analyses of subgroups of children with different but consistent patterns of code-switching confirmed that children who code-switched to English showed greater English skills, specifically in the expressive domain. Children who did not code-switch were more balanced bilinguals in both expressive and receptive skills. Children with other code-switching patterns showed still different profiles of dual language expressive and receptive proficiency. These findings reveal that some, but not all, bilingual children show different profiles of expressive and receptive skill in their two languages and that these proficiency profiles are related to their language choices in conversation.