Background: Over 15% of kindergarten and first grade students in U.S. public schools are English learners (ELs) (National Center for Education Statistics [NCES], 2021). Many classroom teachers, however, have not had specialized training in providing instruction for ELs. The BELLA (Bridging English Language Learning and Academics) professional development program was created to support structured collaboration between English-as-a-Second Language (ESL) and K-1 classroom teachers as they align implementation of high-impact instructional strategies and incorporate families' cultural wealth into the classroom to benefit EL students' language development and literacy achievement. Purpose and Research Questions: The purpose of this study was to examine the efficacy of the BELLA program for improving K-1 teachers' literacy instructional practices for ELs and improving ELs' outcomes in language and literacy. The research questions included: (1) What impact does the professional development program have on kindergarten and first grade classroom teachers' use of high-impact instructional strategies, incorporation of cultural wealth in the classroom, and collaboration processes and frequency as compared to control teachers?; and (2) What impact does the professional development program have on EL students' language and literacy outcomes as compared to ELs in the control condition? Setting and Participants: This study was conducted in 13 elementary schools in two school districts in North Carolina. The percentage of students from economically disadvantaged backgrounds ranged from 26.6% to 70.6%. Forty classroom teachers (18 kindergarten and 22 first grade) along with 13 ESL teachers participated in the RCT during the 2018-2019 school year. School teams were randomly assigned to either the intervention group or the control group. One hundred eight students from participating teachers' classrooms who were eligible for ESL services and spoke Spanish at home participated in the study. Intervention The BELLA professional development program included four days of in-person workshops that focused on: (1) high-impact instructional strategies for promoting language and literacy development for young ELs; (2) a structured framework to support collaboration among classroom and ESL teachers, and (3) approaches for building on students' cultural wealth in the classroom. School teams collaborated each week to align their instructional strategies and content instruction for their ELs. The implementation coach from the research team met with each school team about once per month. Research Design and Data Collection: This study is a cluster randomized controlled trial with randomization at the school level. Teacher outcomes included classroom observations of classroom literacy instruction, observations of cultural wealth in the classroom, and teachers' reports of their collaboration process and frequency. Research staff who did not know the intervention status of the teachers observed classroom literacy instruction using the BELLA Observational Tool and a cultural wealth checklist prior to the professional development (baseline) and three additional times during the school year. The BELLA Observational Tool included 38 indictors of 8 instructional strategies that target both word level and text level skills. Inter-rater reliability was in the moderate range with Cohen's weighted kappa ranging from 0.66 to 0.88. The student outcome measure was the MAP Growth K-2 (MAP) reading assessment (Northwest Evaluation Association, 2019) administered at the beginning, middle, and end of the school year. The total score, called the RIT score, was used as the primary student outcome measure. We also examined students' scores on the four goal areas (or subtests): Foundational Skills (FS), Language and Writing (LW), Literature and Information (LIT), and Vocabulary Use and Functions (VUF). Data Analysis Study hypotheses were tested using repeated measures ANCOVA models that incorporated the following covariates: at the student level, gender; at the school level, proportion of students who identify as female, proportion of teachers with greater than five years of teaching experience, proportion of teachers with a master's degree, proportion of teachers who indicated they speak Spanish, percentage of students at the school identified as economically disadvantaged, and proportion of third-, fourth-, and fifth-grade students on grade level for reading. Focal allowed-for factors included Time (Time 0, Time 1, or Time 2), Grade (Kindergarten vs. First Grade), Condition (Control vs. Intervention), and their interactions. Our data reflected a fully nested, hierarchical design. A mixed modeling approach with restricted maximum likelihood estimation was employed using the SAS procedure MIXED (SAS/STAT software version 9.4). For each outcome, a random intercept was estimated to account for the clustering of students within schools and districts. District was included in all models as a fixed effect. Covariates were group-mean-centered, such that the observed variable score captured the deviation of the original score from the mean score of the cluster to which the sampled unit belonged. To model within-student (R-side) covariation, a series of three models was tested for each outcome, with each model specifying a different candidate covariance structure for the student-level residuals. Information criteria were used to choose the optimal structure for each outcome. After selection of a final model, fixed effect parameters were estimated and interpreted. Findings: A statistically significant Time x Condition interaction effect was observed for the overall RIT score outcome (p = 0.02) and the LIT subtest outcome (p = 0.002). For both outcomes, there was a positive and statistically significant effect of intervention at both Time = 1 and Time = 2 (but not at baseline). The Time x Condition effect was not statistically significant for the FS, LW, or VUF subtests. Conclusions: In this RCT to evaluate the efficacy of the BELLA professional development program, we found a significant impact of the intervention on students' progress in language and literacy skills. In our presentation, we will also describe the impact of the intervention on teacher outcomes. This study provides evidence of efficacy for a teacher PD program for improving instruction and accelerating student growth. Additional research is needed to test mediation hypotheses that address the extent to which students' growth in language and literacy can be explained by changes in teacher outcomes.