This paper is a report on an experimental study which intended to look into the possible effects of Competitive Team-Based Learning (CTBL) vis-à-vis Group Investigation (GI) method of Cooperative Learning (CL) on the language proficiency of Iranian EFL intermediate students. Seventy homogeneous Iranian intermediate students were selected out of a total population of 110 to serve the present study. The results of the study indicated the advantage of CTBL over GI in terms of its effect on improving the target group's language proficiency. The results of the study were in contrast to the reports of researchers like Ab-Raza (2007), an Israeli language specialist, who have argued that students in Islamic countries "do not value diversity of ideas, beliefs, and perspectives" (p. 5) and so cannot be taught through modern methods like those of CL.