Success in higher education depends on students' ability to meet the writing requirements of their chosen courses, and in many cases this involves adapting to the literacy practices of particular disciplines. While research into professional academic discourse suggests that it may reflect differences in disciplinary culture and epistemology, there has been relatively little investigation of disciplinary difference in student writing. This study is based on an analysis of essays written within an Open University course in the history of science, using a systemic functional approach to examine whether the students' use of theme may vary according to their disciplinary background. Students from an 'arts' background were found to achieve significantly higher grades than those from a 'science' background. This could be related to a greater tendency to present knowledge as constructed, using themes which framed the discussion as a matter of interpretation rather than fact. The results support the hypothesis that students' writing is shaped by their disciplinary background, suggesting that success in writing for one course may be affected by writing experiences in previous courses.