Cell genesis in the adult brain of zebrafish, with specific reference to the olfactory bulbs, was examined using bromodeoxyuridine immunocytochemistry. Mature fish were exposed to a 1% solution of the thymidine analog 5-bromo-2'-deoxyuridine for 1 h and then killed after short (4-h) or long (3-4-week) survival periods. A monoclonal antibody to bromodeoxyuridine allowed visualization of cells that incorporated the drug during the S phase of mitosis. Four hours after administration of the drug, antibody-labeled cells were found almost exclusively in the proliferative zones around the ventricles and in the cerebellum. Very few labeled nuclei were seen in other locations in the brain, indicating that cell genesis occurs in discrete regions in adults. The few labeled profiles in the olfactory bulbs were located in the olfactory nerve layer; these profiles had the morphology of glial nuclei and did not stain with a neuronal marker, the Hu antibody. After longer survival times, labeled cells were present throughout the layers of the olfactory bulb, and many of the immunoreactive profiles in the internal cell layer were also labeled with the Hu antibody, indicating that they are likely adult-formed interneurons. Thus, neurogenesis continues in the olfactory bulb of adult zebrafish. Understanding the process of the generation of new neurons in the brain of adult animals can lead to important insights into neural regeneration and adult plasticity.