[3H]Thymidine autoradiography of explant cultures from the spinal cord of an adult teleost, Sternarchus albifrons, reveals the presence of thymidine-labeled cells with neuronal morphology. These cells have also been identified as neurons by positive staining with two monoclonal antibodies against neurofilaments. Thymidine labeling occurs in cultured neurons derived from both normal (histologically and functionally mature) and regenerating spinal cord of adult Sternarchus albifrons. These results provide evidence that some cells in spinal cord of adult Sternarchus retain the ability to incorporate thymidine and undergo neuronal differentiation in vitro.