Effective use of outside source texts is one of the key components of successful academic writing. This study aims at clarifying Japanese university EFL students' citation behaviors in producing argumentative writing. Twenty-six Japanese university EFL students wrote an argumentative essay. Their essays were analyzed quantitatively by six measures: languages of sources, credibility of sources, kinds of sources, citation strategies, references in essays, and reference lists in essays. The results of the analysis revealed that the students tended to (1) rely on Japanese, rather than English, sources; (2) distinguish credible from non-credible sources; (3) favor books and online materials as sources; (4) use data as well as direct quotations and verbatim translations as citation strategies; (5) properly cite sources in their own essays; and (6) list sources in a reference section. A qualitative sample analysis was conducted to examine actual instances of both effective and ineffective citation practices manifested by a student writer. The study suggests that it is necessary to integrate four language skills into writing instruction, to understand each student's citation behavior from the developmental perspective, and to prepare more precise guidelines for the use of online materials.