This study is part of a 5-year professional development intervention aimed at improving science and literacy achievement of English language learners (or ELL students) in urban elementary schools within an environment increasingly driven by high-stakes testing and accountability. Specifically, the study examined science achievement at the end of the first-year implementation of the professional development intervention that consisted of curriculum units and teacher workshops. The study involved 1,134 third-grade students at seven treatment schools and 966 third-grade students at eight comparison schools. The results led to three main findings. First, treatment students displayed a statistically significant increase in science achievement. Second, there was no statistically significant difference in achievement gains between students at English to Speakers of Other Language (ESOL) levels 1 to 4 and students who had exited from ESOL or never been in ESOL. Similarly, there was no significant difference in achievement gains between students who had been retained on the basis of statewide reading test scores and students who had never been retained. Third, treatment students showed a higher score on a statewide mathematics test, particularly on the measurement strand emphasized in the intervention, than comparison students. The results indicate that through our professional development intervention, ELL students and others in the intervention learned to think and reason scientifically while also performing well on high-stakes testing. (Contains 6 tables.)