We describe the role of the Drosophila melanogaster hephaestusgene in wing development. We have identified several hephaestusmutations that map to a gene encoding a predicted RNA-binding protein highly related to human polypyrimidine tract binding protein and Xenopus laevis 60 kDa Vg1 mRNA-binding protein. Polypyrimidine tract binding proteins play diverse roles in RNA processing including the subcellular localization of mRNAs, translational control, internal ribosome entry site use, and the regulation of alternate exon selection. The analysis of gene expression in imaginal discs and adult cuticle of genetic mosaic animals supports a role for hephaestus in Notch signalling. Somatic clones lacking hephaestus express the Notch target geneswingless and cut, induce ectopic wing margin in adjacent wild-type tissue, inhibit wing-vein formation and have increased levels of Notch intracellular domain immunoreactivity. Clones mutant for bothDelta and hephaestus have the characteristic loss-of-function thick vein phenotype of Delta. These results lead to the hypothesis that hephaestus is required to attenuateNotch activity following its activation by Delta. This is the first genetic analysis of polypyrimidine tract binding protein function in any organism and the first evidence that such proteins may be involved in theNotch signalling pathway.