When half a dozen students in Neil Waters’s Japanese history class at Middlebury College asserted on exams that the Jesuits supported the Shimabara Rebellion in 17th-century Japan, he knew something was wrong. The Jesuits were in “no position to aid a revolution,” he said; the few of them in Japan were in hiding.

He figured out the problem soon enough. The obscure, though incorrect, information was from Wikipedia, the collaborative online encyclopedia, and the students had picked it up cramming for his exam.

Dr. Waters and other professors in the history department had begun noticing about a year ago that students were citing Wikipedia as a source in their papers. When confronted, many would say that their high school teachers had allowed the practice.

But the errors on the Japanese history test last semester were the last straw. At Dr. Waters’s urging, the Middlebury history department notified its students this month that Wikipedia could not be cited in papers or exams, and that students could not “point to Wikipedia or any similar source that may appear in the future to escape the consequences of errors.”