An intermediate filament fraction, isolated from goldfish brain, contains a prominent protein having a molecular weight of 51 kDa. In normal goldfish visual pathway, this protein is present in tectum and tract, but not in optic nerve. A polyclonal antibody raised to this protein clearly labels ependymal glial profiles in tectum and parallel processes in the tract, whereas optic nerve is unlabelled; Müller fibres in the retina are also labelled. A similar, but less prominent, pattern of staining is observed with antibodies, raised elsewhere, against glial fibrillary acidic protein from human and porcine. These results suggest that the 51 kDa protein is a GFAP, demonstrate the heterogeneity of astrocytes in goldfish visual pathway, and are consistent with the idea that GFAP is well conserved in vertebrate phylogeny.