Protein gene product 9.5 was immunolocalized in the adult and early postnatal (P2-P15) rat cochlea, and its distribution compared with a 200 kDa highly phosphorylated neurofilament subunit (neurofilament 200) and α-tubulin. In the adult, Protein gene product 9.5 was expressed exclusively in cochlear nerve fibres and ganglion cells, a small percentage of these (Type II ganglion cells and olivocochlear bundle fibres) being intensely positive for both protein gene product and neurofilament 200. In postnatal development, pillar and Deiters' cells were at first (P2-P15) strongly positive for protein gene product 9.5, and hair cells moderately so. At P2, all nerve fibres and ganglion cells showed co-expression of protein gene product 9.5 and neurofilament 200, but at later stages, the subset of intensely co-labelled neurons appeared, nerve fibres at P7 onwards and ganglion cells from P12. There was no overt correlation between the onset of protein gene product 9.5 and a-tubulin expression in any cochlear component. Protein gene product 9.5 expression in ganglion cells was at first (P2 and P7) mainly nuclear, and later also cytoplasmic. It is concluded that there is a clear correlation of high levels of protein gene product 9.5 and neurofilament protein expression, and that protein gene product 9.5 is expressed in some non-neuronal cells of the cochlea during its early development, persisting until after hearing has commenced.