Plagiarism appears to be a common problem among college students, yet there is little evidence on the effectiveness of interventions designed to minimize plagiarism. This study presents the results of a field experiment that evaluated the effects of a web-based educational tutorial in reducing plagiarism. We found that assignment to the treatment group substantially reduced the likelihood of plagiarism, particularly among student with lower SAT scores who had the highest rates of plagiarism. A followup survey suggests that the intervention reduced plagiarism by increasing student knowledge rather than by increasing the perceived probabilities of detection and punishment. (Contains 2 figures, 8 tables and 26 footnotes.)