In this article, the author reports on findings of second and third graders' science writing about Jupiter in order to describe their expressive fluency with scientific discourse. The students' artifacts were created within a setting of integrated science/literacy instruction in which students participated in hands-on science activities, engaged in extended talk around scientific ideas, and read and wrote scientific texts. The author also presents a framework for evaluating the written scientific discourse of second- and third-grade students. This framework essentially offers teachers a language for the assessment and support of students' scientific language acquisition. Its value lies in the potential for educators to use such a framework to support the full participation of all students in the field of science.