The authors used the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K) data to examine the mathematics and science achievement of two immigrant groups in the United States--Chinese and Mexican students. The authors also assessed variation in parental practices and fifth-grade achievement according to ethnicity and the age at which parents arrived in the United States, i.e. comparing children whose parents were born outside of the United States and immigrated after the age of 17, those whose parents immigrated before the age of 17, and native-born parents. The findings support the segmented assimilation framework: Mexican immigrant parents (both arriving as adults and at younger ages) and Chinese immigrant parents (arriving as adults) reported lower levels of parental involvement at school, enrollment in extra-curricular activities, and provision of literacy materials. However, these behaviors among native-born parents of Mexicans and Chinese more closely resemble those of native-born whites. At the same time, although immigrant parents uniformly expressed higher educational expectations than white parents, the achievement patterns of their children diverged along ethnic lines and varied according to the life stage that the parents immigrated to the United States, indicating a process of increasing assimilation among immigrant families but with group-specific patterns. The authors found decreasing disparities in mathematics and science achievement between immigrant and white students: Chinese students' advantage and Mexican students' disadvantage relative to white students disappeared by the second generation and third generation, respectively. (Contains 6 notes, 6 tables, and 1 figure.)