This article explores the use of alternative assessments as a way to determine the impact of science instruction on English Language Learners' (ELLs) abilities to comprehend an informational trade book text, both at the level of reading comprehension and at the level of science understanding. This multi-cohort comparison study of 47 fifth grade ELLs in a large urban district in the southwest compared students' retellings of an informational text on the science topics of wind energy and wind turbines. All students received 14 days of intensive, hands-on instruction on science concepts with the experimental group studying wind energy and wind turbines and the control group studying genetics and DNA. Findings from data analysis reveal that students who received instruction on the science concept of wind energy and wind turbines (experimental group) had more accurate retellings of the related text at the reading comprehension level. Students in the experimental group also demonstrated stronger conceptual understanding of the science concept of energy transfer. This study has implication for science teachers as it provides evidence to support retelling as an assessment method in the science classroom.