Previous studies show that young monolingual children's ability to "fast map" new word forms is closely associated with both their age and existing vocabulary knowledge. In this study we investigate potential relationships between age, fast mapping skills and existing vocabulary knowledge in both languages of developing bilingual preschool children. Participants were twenty-six typically developing children, ages 3 ; 0 to 5 ; 3. All children learned Hmong as their primary home language (L1) and English as a second language (L2). Fast mapping and vocabulary knowledge tasks were administered in L1 and L2. For vocabulary knowledge, scores were comparable in L1 and L2; for fast mapping, scores were somewhat greater in L1 than L2. In contrast to previous findings with monolingual children, fast mapping performance was not related to age or existing vocabulary knowledge in either Hmong or English. There were, however, significant positive and negative cross-language correlations between L1 fast mapping and L2 vocabulary.