In environmental and sustainability education, critical thinking is important for the pluralistic teaching tradition, creating opportunities for student autonomy, deliberation and participation. However, focusing on student participation can also result in relativism or reproduction. Consequently, this article aims to investigate how different epistemological levels in whole-class conversations about sustainability issues can support students' emergent critical thinking. Transcripts of video-recorded whole-class conversations from 11 lessons in primary classrooms were analysed to answer the following research question: What epistemological levels are in play when the aim is to enhance critical thinking, and how are they visible in whole-class conversations? Our analysis reveals that all epistemological levels are essential in supporting primary students' critical thinking, and it shows how teachers can avoid relativism and reproduction through specific teaching moves that also enhance critical thinking. Thus, critical thinking is necessary to achieve the democratic aims of the pluralistic teaching tradition.