We describe here the isolation and characterization of a zebrafish Delta homologue (delta D). A PCR fragment was used to obtain overlapping cDNA clones encoding a protein of 717 amino acids with all characteristic features of proteins of this family, a signal peptide, a transmembrane domain, and an extracellular region comprising the DSL domain and eight EGF-like repeats. The gene is transcribed in a complex pattern in the developing nervous system as well as in the hypoblast. Overexpression of this gene following mRNA injections leads to a reduction in the number of islet-I positive cells, which are assumed to be primary neurons, and to various defects in the adaxial mesoderm, as well as in the somites and myotomes. This suggests that delta D, and the Notch signalling pathway are involved in the differentiation of primary neurons within the neural plate, as well as in somite development.