L1 and L2 summaries written by Korean university students in an EFL writing class were studied to determine the causes of instances of “plagiarism.” Summaries in both L1 and L2 were evaluated holistically along with seven essays in L2. It was found that more proficient writers in L1 and L2 tend to demonstrate less Reliance on the Source Text1); and a good amount of Reliance was also present in L1 as well as L2, suggesting that L2 interference is not the only cause of Reliance. These findings indicate a relation between Reliance on the Source text and writing ability both in L1 and L2. Moreover, a closer look at instances of Reliance often revealed efforts by the writers to integrate source text material into their own texts, though these attempts often seemed like “plagiarism” to writing teachers. However, Reliance in these cases may not be plagiarism per se from students’ point of view. The results are expected to help teachers understand the causes and nature of students’Reliance on the Source text in the writing instruction.