This article addresses the issue of whether bilingual children begin phonological acquisition with one phonological system or two. Five hypotheses are suggested for the possible structure of the bilingual child's phonological system. Analyses of data from a longitudinal study of a Farsi-English bilingual infant, Arsham, supported the hypothesis that the child had acquired two separate phonologies with mutual influence; that is, he made occasional use of phonological features of Farsi in English words and vice-versa. It is suggested that this was due to the pattern of exposure to the two languages, and that other children may show a different pattern, depending on their exposure to the two languages and the role of language dominance.