The traditional distinction between writing across the curriculum and writing in the disciplines (WID) as writing to learn versus learning to write understates WID's focus on learning in the disciplines. Advocates of WID have described learning as socialization, but little research addresses how writing disciplinary discourses in disciplinary settings encourages socialization into the disciplines. Data from interviews with students who wrote lab reports in a biology lab suggest five ways in which writing promotes learning in scientific disciplines. Drawing on theories of situated learning, the authors argue that apprenticeship genres can encourage socialization into disciplinary communities.