The primary purpose of this investigation is to ascertain the long-term effect of a maintenance bilingual education program (K-2) on the acquisition of Spanish (L1) and English (L2) for two principal linguistically homogeneous groups of Mexican-American children who were all limited in English (LEP) as well as Spanish (LSP) proficiency. For purposes of comparative analysis, these program participants were divided into a most-limited-language-proficient group (n = 74) and a limited-language proficient group (n = 113). Although the primary analysis compared L1 and L2 acquisition, the second analysis examined changes in oral English and Spanish for the two principal groups at kindergarten and second grade. The most significant overall results related to the additive effects that the MBE program fostered for the two groups' oral L1 and L2.