The purpose of this study was to analyze the effects of a two-way immersion elementary school program on academic achievement at the end of the elementary school and the end of the first year of junior high school. Longitudinal high stakes test data in reading, writing, and mathematics were collected on native English speakers and native Spanish speakers from the two-way immersion program and on matched controls through the use of an ex post facto quasi-experimental design. Findings suggest consistent support for the two-way immersion program over matched control students across all three achievement areas. It appears the greatest effect for native English speakers may be in reading, while native Spanish speakers may benefit more in writing and mathematics. Limitations to generalizability and causal inferences due to the small sample sizes and inherent weaknesses of the research design are noted. (Contains 3 tables.)