Over the past few decades co-teaching has attracted due attention for enhancing learning process in educational systems. Drawing on the concept of ``parallel model``, the present study attempted to examine the extent to which this approach can affect general language proficiency of EFL learners. To this end, through a quasiexperimental study, a group of 32 second-grade students studying English in junior high school were assigned into a Control Group (CG) and an Experimental Group (EG) according to their pre-test performance. Parallel teaching by co-instructors was implanted in EG context, while for the CG the instruction was delivered by a single teacher. The results of the post-test administration revealed that difference in instruction approaches did not result in a significant difference in the subjects`` performance. Further investigation made it clear that before any attempt in its implementation, learners need to be educated and culturally prepared to benefit from such an approach.
