Background/Context: Literacy stands as a pivotal life skill, especially as it strengthens education across subjects. The elementary years are crucial for mastering literacy, yet national assessments show a decline in elementary students' reading proficiency, with only 33% of American fourth graders reading at grade level (National Center for Education Statistics, 2023). Effective early education can equip children with essential skills such as literacy. Historical programs like Perry Preschool and contemporary ones like Boston's pre-school program have shown significant long-term benefits (Heckman et al., 2010; Gray-Lobe et al., 2023). However, there is limited causal evidence on the lasting impact of early literacy programs. The "trifecta skills" framework suggests lasting effects occur when interventions target fundamental and malleable skills (Bailey et al., 2017). Incorporating science content into literacy programs offers rich language opportunities, which may support early skill development (Barnes et al., 2016; Dwyer et al., 2016; Lawrence et al., 2015). Purpose/Objective/Research Question: This study builds on the prior impact evaluation of Zoology One. The original study assessed its effectiveness on kindergarten students' literacy outcomes and found positive impacts on students' passage comprehension, motivation to read and letter name fluency (Gray et al., 2022). The objective of this paper is to investigate effects heterogeneity by cohort and long-term impacts up to four years after the intervention. Setting: School District of Philadelphia, Pennsylvania. Population/Participants/Subjects: The initial study involved 71 kindergarten classrooms across 21 schools, totaling 1,589 participants (Gray et al., 2022). The study spanned two school cohorts: Cohort 1 comprised 12 schools with 40 kindergarten classrooms during the 2016-17 academic year, and Cohort 2 included and additional nine schools with 31 kindergarten classrooms during the 2017-18 academic year. The analysis incorporated both intent-to-treat (ITT) and treatment-on-treated (TOT) analytical samples. Approximately 8% of the sample were designated as English Language Learners, 7% had an Individualized Education Plan, 66% were classified as economically disadvantaged, 73% were BIPOC, and 50% of the students were female. Intervention/Program/Practice: Zoology One is a kindergarten literacy curriculum that integrates science content. It involves a 90-minute instructional block that includes morning meeting, readers' and writers' workshops using science-based texts and materials, small-group instruction, and hands-on science exploration activities. Instruction is tailored through a data driven approach to formative assessment and leveling. In addition, the curriculum emphasizes the importance of building a culture around literacy and the volume of reading and writing through activities for independent reading, daily writing and science instruction, and home reading. Students in treatment classrooms received Zoology One as the literacy curriculum, and control classrooms experienced business as usual literacy instruction. Teachers in the treatment group received professional development and ongoing classroom-based coaching throughout the year. Research Design: The effectiveness of Zoology One was evaluated through a multi-site cluster-randomized controlled trial (RCT), incorporating randomization at the classroom level with blocking on school (Gray et al., 2022). This design supports causal inferences about program impacts. Baseline was established between treatment and control. The current longitudinal study follows both cohorts over four school years. Data Collection and Analysis: The original study collected a variety of outcomes aligned with Zoology One's theory of change (Gray et al., 2022). These assessments were administered by both the school district and the research team and encompassed the Woodcock Reading Mastery Tests (WRMT), Kauffman Test of Education Achievement (KTEA) for writing, and AIMSweb. The current study examines follow-up outcomes on WRMT and AIMSweb in grades K through 2. In addition to exploring standardized measures, this study focuses on assessing long-term student achievement, measured through quarterly and final course letter grades in reading, writing, and science spanning 1st through 4th grades. Using course grades, we implemented a three-level mixed-effects ordinal logistic model to explore longitudinal impacts (Hedeker & Gibbons, 2006). Longitudinal analyses were followed-up with single year impact analyses to explore impacts on course grades from 1st to 4th grade. Exploratory analysis of treatment effect variation among subgroup of students over time were conducted. Different levels of attrition across grade levels were found. To address this issue, an inverse probability weighting (IPW) approach was used to minimize the bias of group differences at baseline between the retained sample and the sample of students who were dropped out due to missing outcomes in later grades. Findings/Results: The analysis indicates an overall positive average effect of Zoology One on reading, writing, and science over time. Notably, there is a significant positive effect in reading (OR = 1.52, 95% CI [1.18, 1.95], p = 0.004) and science (OR = 1.32, 95% CI [1.03, 1.69], p = 0.028), suggesting that students exposed to Zoo have higher odds of achieving a higher outcome level in reading and science (see Table 1). On average, the odds of Zoo students are 52% higher than those of control students in attaining a higher outcome level in reading, and 32% higher in science. The single-year impact analyses reveal a positive effect of the intervention in 2nd grade (OR = 1.43, 95% CI [1.19, 1.73], p = 0.008), 3rd grade (OR =1.40, 95% CI [1.15,1.71], p = 0.011), and 4th grade (OR = 1.42, 95% CI [1.19, 1.70], p = 0.005) in writing. Heterogeneity analyses revealed larger magnitude treatment effects among cohort 1 students on writing in 2nd, 3rd, and 4th grade (see Table 2). Conclusions: Our study highlights the importance of grades as intermediary outcomes in long-term skill development, especially in early childhood interventions. The timing of these outcomes may be important, shedding light on effects that may dissipate in elementary school only to resurface in adulthood, as seen in other programs like Perry Preschool and the Boston Pre-K program. By demonstrating impacts on course grades, our findings offer insight into comprehensive measures beyond traditional assessments. While challenges persist in measuring non-academic outcomes in early elementary school, our study provides nuanced understanding of the impact of blended literacy curricula without adverse effects.