In light of the striking differences between oligodendrocytes of the optic nerve/tract of adult goldfish and their mammalian counterparts, a further characterization of goldfish oligodendrocytes was performed. A comparison with Schwann cells was included because fish optic nerve/tract-derived oligodendrocytes bear remarkable similarities to this type of glial cell. Fish optic nerve/tract-derived oligodendrocytes that had differentiated into 04 and 6D2-positive cells and thus expressed early myelin marker molecules were found to incorporate BrdU and to divide in vitro over a period of weeks. For the induction of more advanced markers of myelinogenesis such as the CNS-specific myelin protein 36K, oligodendrocytes required extensive contact with axons. Other agents, such as fetal calf or carp serum, substrate components, or forscolin failed, however, to induce 36K expression. 04/6D2-positive oligodendrocytes could be distinguished from fish 6D2-positive Schwann cells derived from cranial nerves by their antigenic phenotype: Schwann cells but not oligodendrocytes exhibited the low affinity NGF receptor. While both cell types carry the cell adhesion molecules NCAM, E 587 antigen, and the L2/HNK-1 epitope, only Schwann cells possess a further adhesion molecule, Neurolin.