This longitudinal case study documents the emergence of bilingualism in a young monolingual Mandarin boy on the basis of an acoustic analysis of his vowel productions recorded via a picture-naming task over 20 months following his enrollment in an all-English (L2) preschool at the age of 3;7. The study examined (1) his initial L2 vowel space, (2) the process of L1-L2 separation, and (3) his L1 vowel system in relation to L2. The child initially utilized his L1 base in building the L2 vowel system. The L1-L2 separation started from a drastic restructuring of his working vowel space to create maximal contrast between the two languages. Meanwhile, L1 developmental processes and influence of L2 on L1 were also in effect. The developmental profile of this child uncovered strategies sequential bilingual children may use to restructure their phonetic space and construct a new system of contrasts in L2.