This study investigated the effect of first language (L1) metalinguistic comments on Iranian EFL learners’ acquisition of third person /-s/ as measured by the learners’ performance on a grammaticality judgment test. Accordingly, two intact university classes- randomly assigned to experimental and comparison groups- were homogenized based on their scores on language proficiency and grammatically judgment tests. During 6 hours of task-based instructions, the experimental group was treated by L1 metalinguistic comments and the comparison group through L2 metalinguistic comments. Using ANCOVA and t-test to compare the groups’ scores, the results indicated that although the mean accuracy scores of both groups were improved on the immediate posttest, the experimental group outperformed the comparison group on the delayed posttest. It can be postulated that L1 metalinguistic comments lead to more durable gains than L2 metalinguistic comments in EFL classrooms. The findings suggest that L1 use may aid language learners to become more cognitively conscious of L2 grammar features.
