The purpose of this study was twofold. One purpose was to identify the locally negotiated literate practices that defined ways of communicating information and knowledge across the curriculum in a fourth/fifth grade classroom. Through an ethnographic and sociolinguistic set of analyses, this investigation illustrated how the teacher worked to construct a learning environment that valued the use of multiple discourses as a way of communicating competence as a learner. Another purpose was specifically to address the ways that these practices contributed to student demonstration of knowledge in science. This was accomplished by an analysis of one student's presentation of science knowledge after a study of simple machines. A comparative analysis between his use of two discourses, written and oral, showed that the opportunity to choose between multiple discourses led to his success in demonstrating competence in ways that may have otherwise remained questionable. © 2004 Wiley Periodicals, Inc. J Res Sci Teach 42: 139–165, 2005