In order to examine the emergence of semantic notions and their interaction with the linguistic codes of bilingual children, this paper focuses on acquisition of a semantic field related to Piagetian tasks of conservation. The hypothesis examined in this study states that the manifestation of a concept will occur first in the bilingual's dominant language and only later in the weaker language. Subjects were two groups of Mexican-American children, first graders and sixth graders, bilingual in Spanish and English. In terms of the school setting, the older bilinguals had shifted language dominance from first to second language; the younger group was developmental, but dominant in first language. Using a cross-sectional approach, the study investigated some of the complex interactions between first and second language cognitive development. The view is taken that the manifestation of certain aspects of conceptual development in the bilingual child's two languages does not involve a simple lateral transfer of a particular concept from one language to the other but, rather, that both languages draw from a shared set of linguistic principles and conceptual notions. In addition to a discussion of the theoretical issues, practical implications for assessment and placement of children in bilingual programs are noted. (Author/AMH)