Fish glial cells were obtained from cultivated segments of the optic nerve and raised in vitro. Two types of cells were identified as astrocyte- and oligodendrocyte-like glia by the monoclonal antibody Mab O1 (specific for oligodendrocytes) and the rabbit serum anti-goldfish glial fibrillary acidic protein (anti-G-GFAP). Cells of compact morphology were rare, and anti-G-GFAP positive and O1 negative. Multipolar cells in 5-day-old cultures were anti-G-GFAP but rarely O1 positive. In 5-week-old cultures, however, roughly 75% of the multipolar cells were double-labeled with both anti-G-GFAP and O1; 10% were single labeled with Mab O1 and 15% with anti-G-GFAP, respectively.