The authors evaluated the effects of P-SELL, a science curricular and professional development intervention for fifth-grade students with a focus on English language learners (ELLs). Using a randomized controlled trial design with 33 treatment and 33 control schools across three school districts in one state, we found significant and meaningfully sized intervention effects on a researcher-developed science assessment and the state science assessment. Subgroup analyses revealed that the P-SELL intervention had a positive and significant effect for each language proficiency group (ELLs, recently reclassified ELLs, former ELLs, and non-ELLs) on the researcher-developed assessment. The intervention also had a positive effect for former ELLs and non-ELLs on the state science assessment, but for ELLs and recently reclassified ELLs, the effect was not statistically significant.