Learning to use language in order to complete source-based writing tasks is a challenge for middle grade writers worthy of additional study, especially given that these tasks are increasingly common in classrooms. Here, we examined the contribution of receptive and productive cross-disciplinary academic language skills to the writing quality of middle graders' science summaries, after accounting for students' sociodemographic characteristics, their comprehension of the science source text, and general summary features (i.e., length and copy ratio). Our results suggest that the ability to produce a high-quality science summary is predominately driven by the writers' receptive academic language proficiency, comprehension of the source text and productive academic language used when writing, including text length. We contend that these results are illuminating because of the joint examination of students' productive and receptive academic language skills. In addition, this study attempts to unpack the relation between summary length and quality, identifying levels of receptive academic language skills as an important moderator. These findings have the potential to advance the current understanding of malleable factors associated with science writing proficiency, as well as with those associated with source-based writing more generally.