The expression and localization of COL2A1 mRNA and protein was examined in human fetal cochlea to study the role of this gene in hearing and to begin to understand the pathogenesis of mutations in COL2A1 in hearing disorders. Northern blot analysis revealed COL2A1 expression in fetal membranous cochlea to be markedly greater than that in fetal skin, kidney, cartilage, eye and brain. In situ hybridization revealed COL2A1 expression in marrow cells, osteoblasts, fibroblasts and some osteocytes, in addition to chondrocytes in otic capsule. In the membranous cochlea, connective tissue elements (spiral ligament, spiral limbus and modiolar connective tissue), neuronal cells, secretory cells (stria vascularis) and organ of Corti cells (sensory hair cells) were found to express COL2A1. Immunohistochemistry was performed to assess distribution of type II collagen and correlation with COL2A1 mRNA in these morphologically and functionally diverse cell populations. In otic capsule, only cartilage was found to stain positively, and in membranous cochlea, only connective tissue structures including spiral ligament, spiral limbus, tectorial and basilar membranes, modiolar and spiral lamina cartilage contained type II collagen. Nonconnective tissue cells, marrow cells and osteoblasts did not contain immunohistochemically identifiable protein. Absence of type II collagen in a subset of cochlear cells may reflect potentially either inability to detect low levels of protein in these cells or posttranscriptional regulation.