Using data from a random sample of Latino students in California, tracked throughout their elementary and middle school years, this paper examines ways in which outcomes vary for students of similar language and socioeconomic background who are initially instructed in their native language in transitional bilingual programs. As the students made the transition to instruction in English, which took place for most students in the sample between Grades 2 and 4, all students experienced an abrupt decline in performance on standardized reading test scores in English. However, performance trajectories after transition took markedly different paths, with higher achievers returning to pre-transition rates of performance as lower achievers continued an achievement decline that began for many while still being instructed in Spanish. The paper also examines school factors potentially associated with variations in performance (school-wide reform and strong kindergarten program) for subsets of the sample. (Contains 5 tables and 2 figures.)