Young children engage in essentialist reasoning about natural kinds, believing that many traits are innately determined. This study investigated whether personal experience with second language acquisition could alter children's essentialist biases. In a switched-at-birth paradigm, 5- and 6-year-old monolingual and simultaneous bilingual children expected that a baby's native language, an animal's vocalizations, and an animal's physical traits would match those of a birth rather than of an adoptive parent. We predicted that sequential bilingual children, who had been exposed to a new language after age 3, would show greater understanding that languages are learned. Surprisingly, sequential bilinguals showed reduced essentialist beliefs about all traits: they were significantly more likely than other children to believe that human language, animal vocalizations, and animal physical traits would be learned through experience rather than innately endowed. These findings suggest that bilingualism in the preschool years can profoundly change children's essentialist biases.