Purpose: A previous investigation (Lee & Iverson, 2012) found that English and Korean stop categories were fully distinguished by Korean-English bilingual children at 10 years of age but not at 5 years of age. The present study examined vowels produced by Korean-English bilingual children of these same ages to determine whether and when bilinguals establish distinct vowel categories across their 2 languages. Method: Both English and Korean vowels produced by 40 Korean-English bilingual children (5 and 10 years of age) were examined in terms of 1st formant frequency (F1) and 2nd formant frequency (F2), vowel duration, and F1 and F2 formant trajectories. Results: Formant frequencies of vowels produced by the bilingual children were similar to those of monolingual English and Korean children. The bilinguals distinguished vowel categories across languages using both the assimilation and dissimilation mechanisms as identified by Flege, Schirru, and MacKay (2003). Conclusions: Vowel categories developed earlier than stops in bilingual children because vowels were typically acquired earlier than consonants. The results of this study suggest that detailed phonetic categories do not form across the board and that bilingual children may invoke multidimensional representations of phonetic categories.