Background: The Framework for K-12 Science Education (NRC, 2012) and Next Generation Science Standards ([NGSS] NGSS Lead States, 2013) emphasize that all students from the earliest grades onward must have the opportunity to learn and actively participate in authentic science. In K-12 science instruction, this means that students should have the opportunity to learn by applying the three dimensions of science and engineering practices, disciplinary core ideas, and crosscutting concepts in the context of reasoning about phenomena and problem solving. Yet, the current reality is there is a lack of emphasis on teaching science in the early elementary grades and the quality of what is being taught varies tremendously, both in content and quality (NAS, 2022). Recently, NGSS-designed instructional materials for the elementary grades have become available to school districts, creating an opportunity for conducting evidence-based research on the efficacy of NGSS designed curricula. We conducted a study of an integrated science and literacy curriculum (ISLC) for first grade classrooms that was initially developed with funding from IES. The curriculum, now a part of the commercially available K-5 Amplify Science program, was designed to support integrated science, language, and literacy learning, consistent with the NGSS. We evaluated the efficacy of ISLC for improving first grade students' learning in relation to NGSS performance expectations and to Common Core State Standards for English Language Arts and Literacy. Two primary research questions motivated this paper: (1) What is the impact of ISLC and accompanying professional learning on students' science and literacy learning? and (2) Does the impact of ISLC on learning vary for students with different prior achievement or other policy-relevant characteristics? Setting and Participants: The study was conducted in 40 ethnically and culturally diverse elementary schools across three districts in California, involving 82 first-grade teachers and 2035 students. One treatment school dropped from the study before the student outcome learning outcome assessments were administered. Each district was committed to providing regular science instruction in the early elementary grades. Table 1 shows the composition of the analytic student sample. Seventy percent of students receive free-or-reduced price lunch (FRPL) and 74 percent identify as Hispanic. Table 1 also shows the results of baseline equivalence tests for the analytic sample. Groups were equivalent at baseline on prior reading proficiency (Hedge g = 0.02) with some minor differences in ethnic composition. Intervention: The ISLC materials consist of three units, each focused on one of three science domains: life, physical, and Earth and space science. Each unit consists of 22 45-minute lessons that can be taught within a 6-8 week period. The curriculum adopts a model of literacy and science integration that embraces a reciprocal relationship: that reading, writing, and language can be employed to promote the learning of science; and that students can make substantial progress in literacy development when reading, writing, and language are used to support science learning. The curriculum includes 3 professional learning (PL) sessions (a total of 4 days) distributed across the school year and timed to prepare teachers to teach each unit. Research Design: Schools were randomly assigned within district to either implement ISLC materials (treatment) or their business as usual materials (control) in their first-grade classrooms. Prior to assignment, schools were matched on enrollment size, percent minority and percent economically disadvantaged students. Teachers in the control group were asked to teach with their regular curriculum materials and participate in typical science professional learning provided by their districts. Control teachers were accountable to the same NGSS performance expectations and coverage of the same Grade 1 science topics as treatment teachers. Outcome Measures: The outcome measures included two standardized Iowa Assessments for Grade 1 Reading and Science and two study-developed NGSS-aligned assessments of science learning and science vocabulary-in-use. All assessments were administered on paper by teachers in the spring towards the end of the school year. Spanish-versions of the assessments were administered by teachers when all their instruction was in Spanish. Further details on the study-developed assessments are included in the Appendix. Data Analysis: We investigated the effect of ISLC on student learning and possible moderators using a multilevel linear regression model (students within schools). Student-level covariates included prior reading scores from fall assessments, gender, English-learner (EL), FRPL and Individualized Education Plan (IEP) status, and ethnicity. At the school-level, the randomization block was the covariate. Results and Discussion: Table 2 shows the results of the impact analysis. We found statistically significantly positive effects of the ISLC curriculum on the study-developed science learning assessment (Hedges g = +0.24) and the science vocabulary-in-use assessment (Hedges g = +0.46). We also found a small positive effect (Hedges g = +0.09) on the science component of the standardized Iowa assessment. This result was somewhat expected given that the Iowa science assessment was not designed for the NGSS and thus less likely to be instructionally sensitive to the use of an NGSS-designed curriculum. Finally, we found no difference between conditions on the standardized Iowa reading assessment. This is an intriguing result. We believe the null effect result for the Iowa reading assessment may be evidence that teachers can teach considerably more science in the early grades using ISLC without concern that students' reading scores will be negatively impacted. We are currently investigating the differences in time spent in science instruction between conditions. None of the effects on learning varied by prior achievement, ethnicity, or FRPL, IEP, or EL status. Conclusions: The NGSS vision for learning requires changes in how science curriculum and instruction are conceptualized, supported, and implemented (Penuel et al., 2015). The results of this study shows promising evidence that curriculum materials designed for the early grades to support NGSS teaching and learning can improve student acquisition of ambitious science skills and science vocabulary usage. As other new-NGSS designed materials become more widely used across geographic regions and with varying student populations, additional studies at larger scale will be needed so that we can further build the evidence base for the uptake of science in the elementary grades.