Instructors communicate what they value about students' written work through their comments and feedback, and this feedback has the potential to direct how students approach writing assignments. In this study, we examined how graduate student teaching assistants (TAs) attended and responded to students' written lab reports in an introductory biology course. We collected and analyzed marked lab reports from five TAs and interviewed them about their marking decisions. The results show that TAs attended mainly to writing style and form in their markings and comments on lab reports. However, there were occasions when they attended to students' scientific reasoning in their markings and during interviews. We provide evidence that TAs' understanding of the purpose of the laboratory course and assessment structure influenced their attention. We also provide evidence that TAs could shift their attention from style and form to reasoning in response to moment-to-moment contextual cues. Building on these results, we discuss course design and professional development that reframes labs and reports to focus on students' biological reasoning.