A class of tectal cells whose mitotic activity is enhanced by optic nerve regeneration in adult goldfish has been identified as radial neuroglia. These mitotic glial cells are morphologically distinct cells with prominent radial processes which extend through the entire depth of the tectal layers. The radial glia incorporate exogenous [3H]thymidine ([3H]TdR) into DNA as early as 2 hr after systemic injection. The plane of cell division for the mitotic radial glia is always aligned to the equator of the cell body, perpendicular to the direction of the radial process. In a few cases, radially adjacent pairs of labeled daughter radial glial cells are observed as early as 12 hr and also as late as 51 days after [3H]TdR injection. In other cases, however, only one labeled daughter radial glial cell is identified, and the other daughter cell cannot be traced. These observations suggest that radial glial cells of the adult goldfish tectum can be induced (presumably by mitogenic effects of regenerating optic nerve fibers) to undergo mitosis.