Dynamic assessment (DA) seeks to understand learners' abilities that are still developing by embedding mediation (i.e., contingent forms of assistance) into the assessment process. To date, DA studies have been carried out primarily in contexts focused on language learning. However, in content classrooms (e.g., science, math), English learners (ELs) are developing content and English language proficiency simultaneously and may therefore be able to demonstrate their learning more fully with mediation than in independent performance alone. This study applied DA to a novel context--the science classroom--and examined whether embedding mediation (in the form of contingent questions/probes) into two science assessment tasks yielded more complete and accurate information about what fifth-grade ELs and their peers knew and could do. Findings indicated that mediation supported ELs in clarifying their science ideas that were initially communicated inexplicitly or imprecisely. Mediation also revealed that some non-ELs' seemingly accurate initial responses were not underpinned by science understanding. By offering a theoretically grounded approach to assessment that is dually responsive to students' content and language learning needs, DA could help refute deficit views of ELs in the content areas and create more equitable opportunities for these students to demonstrate their content learning.