A Rosenthal fiber is a thick, elongated, worm-like or "corkscrew" eosinophilic (pink) bundle that is found on staining of brain tissue in the presence of long-standing gliosis, occasional tumors, and some metabolic disorders.

Its presence is associated with either pilocytic astrocytoma^[1] (more common) or Alexander's disease (a rare leukodystrophy). They are also seen in the context of fucosidosis.

Rosenthal fibres can also be seen in craniopharyngioma.

The fibers are found in astrocytic processes and are thought to be clumped intermediate filament proteins, primarily glial fibrillary acidic protein.^[2] Other reported constituents include alphaB crystallin, heat shock protein 27, protein beta-1), ubiquitin, vimentin, plectin, c-Jun, the 20 S proteasome, and synemin.^[3]