The results of a longitudinal evaluation of an innovative educational program in bilingual immersion at the San Diego City Schools are presented. The program's goals emphasized the acquisition of communicative proficiency, in English and Spanish, for both native-English-speaking and native-Spanish-speaking students. Using a quasi-experimental longitudinal design, the evaluation investigated the effectiveness of the San Diego program on the academic achievement, in both Spanish and English, of 78 fluent-English-proficient students (FEPS) and limited-English-proficient students (LEPS) in grades 2 through 6. As expected, both groups of students demonstrated achievement gains in reading and math (on both Spanish- and English-language tests), and performed at or above national norms in all subject areas by the end of elementary school. These findings are discussed in terms of their conceptual and evaluative significance, and with respect to ongoing debates in the scholarly and policy-making communities. 36 references. (Author/DJD)