This study is a detailed analysis of the phonological development of Sylvia, a bilingual child, in her acquisition of American English and Brazilian Portuguese from the age of 1 year, 7 months to 3 years, 8 months. The study is divided into four stages: ages 1.7 - 1.9; 1.9 - 2.1; 2.1 - 2.3; and 2.3 - 2.8. Up to the age of 1.9, the same phonological substitutions applied to Sylvia's production of phonetically identical segments in both English and Portuguese. After the age of 1.9, phonological differentiation developed, and with it a distinct set of substitutions became associated with each language. Although this study involves only one child, it does suggest that child phonology can reveal significant things about the adult system. It also suggests that the two languages have quite different trends for future change. Evidence for this can be found from dialect comparisons as well as from Sylvia's different treatment of the two languages. Perhaps a language's tendency toward a specific type of change is governed by a language specific speech posture and a specific set of phonological processes which is a natural consequence of such a posture. (Author/CFM)