Previous research in the USA studying Spanish-English bilingual children's language development has largely focused on children's developing abilities in Spanish. However, relatively little research has been conducted on children's English grammatical development. Therefore, the purpose of this study was to examine the English grammatical development of bilingual (Spanish-English) Head Start children during the preschool years. The goals were to determine (a) whether there are differences in children's productions of English grammatical morphemes based on timing of English exposure and (b) which morphological structures met mastery, emerging and early emerging levels of production by bilingual children. These aims were accomplished through in-depth analyses of spontaneous language samples collected over a two-year period in Head Start. Comparisons were made between Spanish-speaking children who were exposed to English at home from birth (home English communication (HEC)) and Spanish-speaking children who were not expected to communicate in English until their entry into Head Start (school English communication (SEC)). Results indicated that children in the HEC group mastered more morphemes earlier than the children in the SEC group; however, by the end of children's second year in Head Start both groups had mastered a similar number of morphemes. Additionally, the children in both groups differed in which morphemes were mastered at the end of Head Start when compared to monolingual English-speaking children. The results of this investigation provide information to clinicians about typical English grammatical development of Spanish-English preschool children. (Contains 8 tables.)