Two case studies of students learning to write academic discourse, particularly to maintain cohesion and relevance, are described. The two college students were part of a fourth-year biology class: one was a native English speaker, and the other spoke English as a second language. Both had a good command of spoken English grammar at the sentence level. The case studies address the following issues: violations of relevance rules for the grammar of written academic discourse, the students' means of acquiring these rules, and the relation of acquisition to error. Numerous samples of compositions produced by these students for the biology course are analyzed. It was found that the students most often had problems with relational predications in their essays. Models that are close to the skill level of the student, insight, and practice are all essential to the acquisition of the rules of discourse. Finally, acquisition of these rules was found to be accompanied by vocabulary errors, mixed constructions, and tense inconsistency. (RW)