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Background: Inner ear morphogenesis is tightly regulated by the temporally and spatially coordinated action of signaling ligands and their receptors. Ligand–receptor interactions are influenced by heparan sulfate proteoglycans (HSPGs), cell surface molecules that consist of glycosaminoglycan chains bound to a protein core. Diversity in the sulfation pattern within glycosaminoglycan chains creates binding sites for numerous cell signaling factors, whose activities and distribution are modified by their association with HSPGs. Results: Here we describe the expression patterns of two extracellular 6‐O‐endosulfatases, Sulf1 and Sulf2, whose activity modifies the 6‐O‐sulfation pattern of HSPGs. We use in situ hybridization to determine the temporal and spatial distribution of transcripts during the development of the chick and mouse inner ear. We also use immunocytochemistry to determine the cellular localization of Sulf1 and Sulf2 within the sensory epithelia. Furthermore, we analyze the organ of Corti in Sulf1/Sulf2 double knockout mice and describe an increase in the number of mechanosensory hair cells. Conclusions: Our results suggest that the tuning of intracellular signaling, mediated by Sulf activity, plays an important role in the development of the inner ear. Developmental Dynamics 244:168–180, 2015. © 2014 Wiley Periodicals, Inc.