Historical trends in plagiarism are often estimated by comparing survey results from a diverse range of samples, institutions, and measures. However, in multi-institution multi-method comparisons changes over time are difficult to separate from differences in methods. We assessed self-reported engagement in, awareness of, and attitudes toward seven forms of plagiarism in surveys of four comparable groups of students at the same university on four occasions over 15 years, each separated by 5 years (2004, 2009, 2014, and 2019). The new 2019 sample was 1099 students. A downward trend in plagiarism from 2004 to 2014 was not continued in 2019, with similar rates of engagement in, awareness of, and attitudes toward the seven forms of plagiarism in 2019 as in 2014. These results emphasize the need to continue efforts to detect and prevent plagiarism, and to educate academics and students about academic integrity conventions.