Is parental language mixing related to vocabulary acquisition in bilingual infants and children? Bilingual parents (who spoke English and another language; n = 181) completed the Language Mixing Scale questionnaire, a new self-report measure that assesses how frequently parents use words from two different languages in the same sentence, such as borrowing words from another language or code switching between two languages in the same sentence. Concurrently, English vocabulary size was measured in the bilingual children of these parents. Most parents reported regular language mixing in interactions with their child. Increased rates of parental language mixing were associated with significantly smaller comprehension vocabularies in 1.5-year-old bilingual infants, and marginally smaller production vocabularies in 2-year-old bilingual children. Exposure to language mixing might obscure cues that facilitate young bilingual children's separation of their languages and could hinder the functioning of learning mechanisms that support the early growth of their vocabularies.