Prompted by a concern for the appropriateness of the modes of discourse as a teaching device and in an attempt to elaborate on the registers of writing required of undergraduate students, case studies were made of six college instructors--two each from biology, English, and sociology. The six teachers were interviewed on two occasions, from 1.5 to 3 hours in each session. During the first interview the instructors were asked to discuss a course that they had just taught, or were planning to teach, for students who were beginning majors, but advanced enough to have completed the basic writing requirements of the institution. Documents--in the form of course syllabi, assignment sheets, and student papers--were also solicited. During the second interview, instructors were presented with a tentative model of the role of written language in the course and asked to confirm, reject, or revise the model. The findings suggest that the teacher's reading of student writing is structured around communicative registers. These semantic patterns are conditioned not only by the customary way of speaking, but also by the instructor's view of what the student should be learning. From the biologists' point of view, the basis for student learning is grounded in certainty. In sociology, the emphasis is on the uncertainty produced by the conflict over sociological knowledge. And English instructors seem to base student assessment on their subjective evaluation of the student as an individual reader. (HOD)