In this study, we adopted the methodology of conversation analysis and interactional linguistics to investigate the use of shiranai and wakannai ‘I don’t know’ in data from (the Corpus of Everyday Japanese Conversation), a corpus of naturally occurring conversations. The quantitative and qualitative analyses of the sequential positions and the interactional functions of these two expressions revealed that shiranai is often used as a response to a question that explicitly requests knowledge status. Shiranai is also used in response to informing, soliciting more talk from the interlocutor. On the other hand, wakannai is more often used as a response to a question that requests information. Used with prosodic features such as elongation and laughter, both shiranai and wakannai sometimes convey the speaker’s affective stance towards the lack of knowledge. Furthermore, wakannai can be used as a discourse marker to mark the speaker’s low degree of commitment, sometimes working as a sequence-closing device.