This study evaluated how effective an online academic integrity module was at reducing the occurrence of plagiarism in a written assignment for a university course. In a preintervention comparison group, plagiarism was detected in 25.8% of papers submitted, compared with only 6.5% in the group that completed the academic integrity module. The grades for plagiarized papers were significantly lower than grades for nonplagiarized papers, suggesting that a substantial amount of plagiarism occurs among students who produce relatively poor quality work. These findings indicate that a substantial proportion of plagiarism encountered was the result of inadequate knowledge about proper quotation and citation. The self-instruction module successfully reduced the occurrence of plagiarism. (Contains 3 notes.)