The ubiquitin A-52 residue ribosomal protein fusion product 1 (UbA52) is a gene highly expressed specifically in the inner ear. Through cellular localization we immunocytochemically investigated its function in the inner ear. In the adult mouse, UbA52 protein was distributed in the strial marginal cells and vestibular dark cells, which regulate the endolymphatic ion homeostasis. In the developing mouse cochlea, no significant staining was observed from birth to postnatal day 3, whereas after postnatal day 6, strong UbA52-immunoreactivities were observed in strial marginal cells. Endolymphatic K concentration is elevated between postnatal days 3-8: therefore, our results indicate that UbA52 may have a functional role in regulation of ion secretion in the inner ear.