This research study explores the role of social annotation in supporting dialogic teaching in secondary English language arts. Grounded in Bakhtin's dialogism and building upon research into online discussion, this study describes how a high school English teacher and her students used a digital annotation tool to read and talk about texts. Analyzing student annotations based on discourse features associated with comprehension and high-level thinking, the study examines the extent to which social annotation supports quality dialogue. Findings highlight the need for open-ended prompts and teacher scaffolding of online discussions, and authors suggest that dialogue enhances comprehension of texts when students go beyond reporting on others' thoughts and instead share their own ideas, connections, and questions about the text.