The authors investigated the influence of engaging in a problem-based learning unit on middle school students' epistemic beliefs, and how such students' epistemic beliefs and approaches to argumentation within and outside of their small groups related. Data sources include state science achievement test scores, epistemic beliefs pre- and posttests, videotaped class sessions, retrospective interviews, and pre- and post-cognitive interviews. Quantitative data were collected and analyzed from 59 students, while the qualitative subsample consisted of 15 students. Engaging in problem-based learning led to a significant effect on students' epistemic beliefs. The effect was of a large magnitude among high-achieving students, of a small magnitude among average-achieving students, and of a small negative magnitude among lower-achieving students. Students employed different approaches to generating and evaluating arguments in different ecosystems, including as small groups and in discussions with the teacher.