In contradiction to previous results, under certain conditions, it is possible to demonstrate GFAP immunoreactivity in the brain and spinal cord of a jawless vertebrate. The critical parameters appear to be (1) fixation with 4% paraformaldehyde at pH 7.4, (2) a very short period (2-6 hr) of postfixation, and (3) immunovisualization by the avidin/biotin/peroxidase technique rather than immunofluorescence or PAP techniques. Immunoreactivity appears throughout the brain and spinal cord, most frequently as fine prolongations normal to the pial surface, which can on occasion be traced to cell bodies near the ventricles or in the ependymal layer. The evolutionary implications of the presence of glial fibrillary acidic protein in a member of the Agnatha are discussed.