Current concerns about academic plagiarism in student writing assume qualitative and quantitative differences in the writing of students for whom English is a first language (EL1) and English is a second language (EL2), but lack precision in measuring those differences. I examined the citation practices of EL1 and EL2 students in a timed writing proficiency assessment essay by applying Web-Based Interactive ESL Text Evaluation System (WIETES) software to identify borrowed texts by classifying these references in 16 bounding and documenting categories on a Reading Reference Grid. Results showed few differences overall between EL1 and EL2 referencing practices, as all students shared three common weaknesses in citation practice: vague references, patchwriting and confusion of quotes and paraphrases. Moreover, all students seemed to go through four stages in their development of coherent citation practices, although no EL2 students functioned at the highest level of citation practice. Interviews with the students about their referencing practices indicated that both groups claimed ownership of ideas from the source materials which they expressed in their own words, and they cited correctly more often when they had prior knowledge of the topic of the reading. I conclude with a call to administrators of writing assessments to include judgements of bounding and documenting practices in analytic rating scales. (Contains 4 tables.)