By using the methodological framework of conversation analysis, this study combines a grounded study of interactional linguistics focusing on L2 speakers' interactional competence in the use of Korean sentence-ending suffix -ketun. Based on the previous studies which examined the use of -ketun in L1 Korean speakers’ conversation, this study analyzes data taken from different levels of Korean language classroom interactions, with close attention to the learner's usage of the target suffix in formulating a dispreferred response. The findings of the study demonstrate that the L2 advanced students' use of -ketun is much closer to that for L1 speakers in constituting a dispreferred response while claiming epistemic primacy. On the other hand, extremely low frequency in the use of -ketun is a distinctive linguistic-pragmatic features that characterizes use of -ketun by L2 speakers at lower levels of competence. The advanced students' diversification may be an indicator of their more adaptive, context-sensitive conduct, suggesting an increased interactional competence. Close scrutiny of the use of -ketun in this study aids to reveal whether and to what extent its interactional use by L2 speakers of different proficiency levels approaches that of L1 speakers.