This study examines the effectiveness of cooperative learning (CL) in developing English as a Foreign Language (EFL) students' grammatical competence in a relatively under-researched context (i.e., Saudi Arabia) and accounts for this effectiveness with reference to students' behaviours, verbal interactions, and their perceptions about learning English lessons in a CL environment. To identify the effect of CL in comparison to traditional small groups on EFL learners' achievements, behaviour states, and verbal interactions, a twelve week study was conducted in four government secondary schools in an EFL context. The participants in this study were 139 male students in the tenth grade, aged 14 to 15 years, in four boys' secondary schools in Al-Baha City, Saudi Arabia. Each school was randomly assigned to one of two conditions, either an experimental or a control group. The researcher videotaped eight EFL classes over a twelve week period: four classes under experimental conditions who are trained in CL principles and skills, and four classes in the comparison groups without this training.