This quantitative, causal-comparative ex-post-facto study primarily examined differences in mathematics achievement between Latinx English Learner (EL) fourth-grade students instructed in English and Spanish within one-way dual/developmental additive elementary bilingual language programs but tested in English. Additional analyses examined the effects of testing in English and Spanish among students instructed in Spanish, as well as the impact of gender and socioeconomic status (SES) on the overall findings. Extant data from 276 Latinx EL students from six different schools in three suburban school districts in Illinois were analyzed. Spring Illinois Assessment of Readiness (IAR) and Measures of Academic Progress (MAP) assessment math scores from 2017-2018 and 2018-2019 were used. Data were analyzed using one-way and two-way analyses of covariance (ANCOVA), with ACCESS English proficiency scores used as a covariate. These analyses found that when tested in English, adjusted mean spring MAP and IAR scores were statistically significantly higher for Latinx EL students' given instruction in Spanish compared to those instructed in English. Conversely, no statistically significant differences in adjusted mean spring MAP or IAR math scores were found between students tested in English and Spanish but instructed in Spanish. No differences among instruction/testing types were observed between genders or levels of SES for either spring MAP or spring IAR. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]