The idea of teaching a target language via a monolingual medium of instruction in the classroom has long predominated in the pedagogical context. In Saudi Arabia, excluding the students’ mother tongue (Arabic) in the foreign language classroom has been seen as a tool that accelerates the acquisition of the target language (English). This is widely viewed as the most practical and effective method of language learning, especially in the Gulf region, where English is a foreign language employed in the fields of economics and business. The recent academic argument that exploiting the students’ linguistic repertoire, including the mother tongue, in the target language classroom boosts and fosters the students’ learning cycle is still encountering huge resistance, especially among second/foreign language teachers. To explore this dispute from the perspective of English as a foreign language (EFL) teachers in intermediate and secondary schools, a case study was conducted with 34 teachers in the Qassim region, Saudi Arabia, through questionnaires and a focus group interview. The study found that most teachers believe that the policy of using the target language (English) only is the most effective method of language learning. They employed the students’ mother tongue (Arabic) on an ad hoc basis to ensure complete comprehension, organize classroom tasks or convey personal remarks. In addition, the study revealed that teachers’ understanding of plurilingualism was unclear and limited to the verbal use of two languages, and that EFL teachers need more clarification on its application in the classroom.