As part of a 5-year professional development intervention aimed at improving science and literacy achievement of English language learning (ELL) students in urban elementary schools, this study examined fourth-grade students' science achievement across a 3-year (2005–2008) implementation of our professional development intervention consisting of curriculum units and teacher workshops. Analyses were conducted with 1,758 students at six schools that participated in the intervention. The results of this study reveal several central findings. First, students in the treatment group displayed a statistically significant increase in science achievement from pre- to posttest. Second, there was no statistically significant difference in achievement gains between students at English to Speakers of Other Languages (ESOL) Levels 1–4 and students who had exited from ESOL or never been in ESOL. Third, there was no significant difference in achievement gains based on the year of participation in the intervention. Finally, students in the treatment group scored substantially lower on National Assessment of Educational Progress (NAEP) and Trends in International Mathematics and Science Study (TIMSS) items than the norm group at the pretest, but higher than the norm group at posttest. These results indicate that ELL students' improved achievement on the science test was a result of effective inquiry-based science instruction as promoted in the professional development intervention. © 2011 Wiley Periodicals, Inc. Sci Ed95:771–793, 2011