This article examines whether bilingual children start phonological acquisition with one or two phonological systems, proposing five hypotheses for the possible structure of the bilingual child's phonological system. Longitudinal analyses of phonological acquisition in a Farsi-English bilingual child, reveal the acquisition of two separate phonologies with mutual influence; it is suggested though that other children may show a different pattern, depending on their exposure to the two languages and the impact of language dominance.