In Drosophila, the Wingless and Notch signaling pathways function in many of the same developmental patterning events. Genetic analysis demonstrates that the dishevelled gene, which encodes a molecule previously implicated in implementation of the Wingless signal, interacts antagonistically with Notch and one of its known ligands, Delta. A direct physical interaction between Dishevelled and the Notch carboxyl terminus, distal to the cdc10/ankyrin repeats, suggests a mechanism for this interaction. it is proposed that Dishevelled, in addition to transducing the Wingless signal, blocks Notch signaling directly, thus providing a molecular mechanism for the inhibitory cross talk observed between these pathways.