In the current study, we examined the effect of cognitive apprenticeship with contextualized language instruction on students with LD (SWLDs) and English learners' (ELs) ability to compose scientific explanations using a multiple-probe, multiple-baseline single-case design. Six middle school students (three in each subgroup) participated in ten 30-min sessions in an after-school program. The intervention was implemented with high fidelity, and all participants believed they benefited from the writing instruction. After instruction, SWLDs and ELs had similar gains. They wrote better explanations, as demonstrated by substantial improvements in the quality of their causal and mechanistic reasoning ([tau] = 1), grammatical and lexical sophistication ([tau] = 1), and holistic writing ([tau] = 0.95-0.96). Changes in writing performance were maintained for 4 or more weeks. We discuss implications of these findings for science classrooms and suggest areas for future writing research.