This paper considers the problem of source code plagiarism by students within the computing disciplines and reports the results of a survey of students in Computing departments in 18 institutions in the U.K. This survey was designed to investigate how well students understand the concept of source code plagiarism and to discover what, if any, specific aspects might cause particular confusion. An analysis of the results was carried out to assess understanding by topic and to discover whether various demographic factors may have an influence on that understanding. Within the survey sample, it appeared that the demographic factors tested did not generally affect students' understanding of source code plagiarism. However, analysis of the data for specific topics revealed that there are several areas of activity where the boundary between acceptable and unacceptable behavior is not clearly understood. These findings have implications for plagiarism education programs. (Contains 2 tables and 1 figure.)