The library literature on plagiarism instruction focuses on students' understanding of what plagiarism is and is not. This study evaluates the effect of library instruction from a broader perspective by examining the pre- and posttest (instruction) levels of students' perceptions toward plagiarism ethics. Eighty-six students completed a pre- and posttest survey that measured their ethical perceptions of plagiarism scenarios. The survey used the multidimensional ethics scale (MES) developed by Reidenbach and Robin that is used commonly in business ethics research. The study found that the MES is a reliable tool to measure changes in ethical perceptions of plagiarism. Further, results indicate that students had higher posttest perceptions of plagiarism ethics than they did prior to library instruction. These results suggest that library instruction was effective and had a meaningful impact on students' perceptions toward plagiarism ethics.