Mutations in the Notch (N) gene affect many processes during the development of Drosophila. Some alleles of Notch affect only particular processes, suggesting that the Notch protein (N) has more than one functional domain. We have found a series of N mutant phenotypes that are similar to those produced by the loss of function of the segment polarity gene wingless (wg) in the embryo and the adult. In addition, we observe that mutations in N behave as dominant enhancers of wg and that there exist allele-specific interactions between the two loci. Our results suggest that the product of wg is a ligand for N.