Informal reasoning fallacies are violations of critical discussion norms. As epistemological understanding of knowledge justification appears to underlie the informal reasoning skills of argument construction and evaluation, it was hypothesized that adolescents with greater epistemological sophistication would be more able to identify informal reasoning fallacies. It was hypothesized that 11th graders would be more epistemologically sophisticated than 7th or 9th graders and, thus, would more likely identify fallacies. Students responded to questions regarding argument scenarios that did or did not contain fallacies. More 11th graders identified fallacies. Epistemological level predicted only identification of one type of fallacy that might be described as epistemological in nature. Cognitive ability also seemed to contribute to the increased ability with grade to identify fallacies.