The purpose of this paper is to determine how the Japanese interjection iya is used and the function it serves in Japanese conversation. This interjection, commonly used in Japanese daily conversation, has usually been explained as expressing a negative response to a previous utterance. However, in a conversation where participants assess something, iya seems rather to fulfill an epistemic function—conveying knowledge claims in and through turn-at-talk and sequences of interaction. This paper demonstrates how each participant asserts his epistemic primacy by uttering iya and explains how the utterance differs from a normal negative response through a conversation/discourse analysis of Japanese conversations for assessment.