With a growing number of Asian students attending Western universities, the difficulties they seem to face in adapting to a new academic environment has provoked much discussion amongst educators, particularly with regard to the critical thinking (CT) skills. Many educators have claimed that, as a result of their cultural and educational backgrounds, Asian students lack the CT skills essential for academic tasks such as essay writing and debates. Other researchers, however, have argued this is due simply to the disadvantages of carrying out studies in a foreign language. In fact, there have been surprisingly few studies directly comparing Asian students' CT skills in their first compared to their second languages. Those that have been done have tended to employ standardised CT tests which, in their discrete, short-answer format, do not accurately reflect the tasks students carry out in university courses. In this study, therefore, two classes of Japanese university students, all with TOEFL scores high enough to enter Western universities, were asked to carry out an oral and written debate, one class in Japanese and the other in English. Evaluations of their performances by independent raters revealed stark differences between the two classes in their ability to construct and deconstruct arguments, find logical inconsistencies and express themselves clearly and persuasively.