The present study aimed to examine how co-teaching may affect the learning process. To do so, a group of 58 first-grade students were assigned to two homogeneous classes. In one group learners received grammar instruction from co-teachers while in other grammar instruction was delivered by a single teacher. The findings revealed that the difference in method of grammar instruction did not lead to significant difference in participants' performance. This finding implies that the appropriateness of co-teaching in educational systems, at least for teaching grammar in an EFL context like Iran, is doubtful and that co-teaching classes should be held cautiously.