As part of a three-year curricular and professional development intervention focused on English language learners (ELLs), this study examined the intervention's effect on teachers' science knowledge and instructional practices after one year of implementation. The P-SELL (Promoting Science Among English Language Learners) intervention comprised curriculum materials for students and teachers and teacher professional development workshops during the summer and throughout the school year. Using a cluster randomized controlled trial design, the study involved 103 fifth grade teachers from 33 treatment schools and 116 fifth grade teachers from 33 control schools across three school districts in one state. The teachers completed a researcher-developed science test and a questionnaire about their instructional practices at the beginning and end of the year. Results indicated a positive effect of the P-SELL intervention on teachers' science knowledge and all four measures of instructional practices: teaching for understanding, teaching for inquiry, language development strategies, and home language use. This positive effect could be attributed to consistent implementation of the key features of the intervention (i.e., standards-based, inquiry-oriented, and language-focused) for all students and ELLs in particular through educative curriculum materials and professional development.