Interspecies marriage (irui kon'in) has long been a central theme in Japanese literature and folklore. Frequently dismissed as fairytales, stories of interspecies marriage illuminate contemporaneous conceptions of the animal-human boundary and the anxieties surrounding it. This dissertation contributes to the emerging field of animal studies by examining otogizoshi (Muromachi/early Edo illustrated narrative fiction) concerning relationships between human women and male mice. The earliest of these is Nezumi no soshi ("The Tale of the Mouse"), a fifteenth century ko-e ("small scroll") attributed to court painter Tosa Mitsunobu. Nezumi no soshi was followed roughly a century later by a group of tales collectively named after their protagonist, the mouse Gon no Kami. Unlike Nezumi no soshi, which focuses on the grief of the woman who has unwittingly married a mouse, the Gon no Kami tales contain pronounced comic elements and devote attention to the mouse-groom's perspective. By elucidating the contrast between Nezumi no soshi and the earliest Gon no Kami manuscript and tracking the development of subsequent versions of Gon no Kami, I demonstrate mounting disenchantment with the irui kon'in trope as a means of telling stories about mice. Tales of interspecies marriage often end tragically; however, in fiction about mice, audience interest came to center on the utopian aspects of the imaginary mouse realm. Thus, mouse-human romance was displaced by storylines more conducive to happy endings, as in mid-seventeenth-century otogizoshi like Yahyoe nezumi ("The Mouse Yahyoe") and Kakurezato ("The Hidden Village"), or slightly later kusazoshi (woodblock-print books) like Nezumi no yomeiri ("The Mouse's Wedding"). The works above belong to a larger body of fiction about mice produced from the late Muromachi to mid-Edo. Previously, mice had received scant literary attention in irui kon'in tales and elsewhere. The sudden boom of "mouse tales" was driven by increased rodent-human contact due to urbanization, and also by the growing popularity of the god Daikokuten, whose iconography prominently featured mice. Mice were simultaneously reviled as vermin and celebrated as good omens, compelling the Gon no Kami stories and other "mouse tales" to negotiate between these contradictory identities.