The study examined to which extent EFL students' language proficiency is related to text quality, lexical profile, and source use (specifically, the amount and type of textual borrowing) in an integrated summary and response task. Student essays from 42 EFL second-year college English-majoring students using a source text were analyzed with identification of summary, paraphrase, reformation of original source text strings, and verbatim copying which were measured with the exact string length of each citation. These essays came from a more proficient group (N=14) and a less proficient group (N=28). Besides documenting citation features, the text quality was also analyzed with their lexical frequency profiles compiled using the VocabProfile tool. Findings indicate that the more proficient group wrote longer essays with better rated quality and more complex vocabulary. They also demonstrated a slightly better skill in incorporating the source text into their essays. It seems that good source using skills required in such a summary-response genre grow with increase of the general language proficiency which involves more sophisticated word use and better and more fluent writing performance. However, a variety of misattribution problems were found in the two groups. Pedagogical implications and future research are discussed against relevant previous studies.