The purpose of this study is to explore the statistical nature of the linguistic interdependence hypothesis (Cummins, 1976, 2017) in the spring of 2nd grade of students in foreign language immersion programs. Specifically, the interdependent relationship between L2 language proficiency levels and L1 reading academic performance in an environment where L1 is the majority language and L2 is the target language for instruction with students who have yet to receive formal reading instruction in the L1. The Continua Model of Biliteracy was used to frame the existing research on language acquisition. An experimental "ex post facto" design using archival data from 228 students in a Spanish immersion program in the Midwest was used. A Spearman rank correlation was used to compare L2 proficiency (STAMP 4Se assessment) to L1 reading academic performance (MAP K-2 reading assessment). The findings of this study showed a strong relationship between L1 reading academic performance and L2 composite scores, L2 reading proficiency, and L2 listening proficiency scores, respectively. A moderate correlation between L1 reading academic performance and L2 writing scores was established, as well as a weak correlation between L1 reading academic performance and L2 speaking proficiency. The researcher concluded that programs and teachers can use L2 proficiency composite and interpretive proficiency indicators as a way to make instructional decisions and monitor growth prior to formal L1 instruction and/or between and before L1 reading high stakes assessment periods. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]