We describe here the primary structure of a zebrafish homologue of the Drosophila neurogenic gene Notch and its pattern of mRNA accumulation during embryogenesis. The gene produces a 8.5 kb transcript encoding a putative transmembrane protein with a high degree of sequence similarity to members of the Notch family, comprising 36 EGF-like repeats, three lin-12/Notch repeats, six cdc10/SWI6 repeats, OPA repeats and a PEST sequence. Transcription of the zebrafish Notch gene is spatially and temporally regulated. A high density of transcripts, most probably of maternal origin, can already be detected in the 2-cell stage. During pregastrulation stages, RNA is present in all cells. However, following gastrulation, transcripts accumulate in specific regions of the embryo following a rapidly changing pattern. In some of these regions, cell divisions take place at the time of Notch expression, in others processes of cell differentiation. This holds true for various mesodermal derivatives, such as the prospective notochord, and for different neural primordia, such as the neural plate and the brain vesicles. This pattern of transcript accumulation suggests a role for the zebrafish Notch homologue in processes of regionalization and cell diversification.