The adult fish brain undergoes continuous neurogenesis and retains the capacity to regenerate. However, the cellular and molecular basis of this process is not well understood. We report on the cloning and characterization of a Brain-1-related, class III POU domain gene, tai-ji, in the developing and adult zebrafish, as well as in a human cell line, hNT2. During development, as differentiation occurs, the expression of tai-ji is downregulated in the notochord, muscle, nervous system and dorsal fin. Similarly, tai-ji is expressed in the human neuronal precursor cell, hNT2, but is downregulated upon differentiation with retinoic acid. In the adult zebrafish nervous system, tai-ji persists in germinal zones, including cells in the germinal zone of the retina, the basal cells of the olfactory epithelium and cells of the subependymal zones in the optic tectum and telencephalon. Subsets of the tai-ji-expressing cells in these regions incorporate BrdU. Most of the tai-ji-expressing cells within these regions of the zebrafish brain are not differentiated and do not express a marker for post-mitotic neurons, acetylated tubulin nor do they express a marker of glial cells, glial acidic fibrillary protein (GFAP). We propose that the majority of the tai-ji-expressing cells are neural stem or progenitor cell populations that may represent the cellular basis for continuous growth in the adult nervous system.