We describe the molecular characterization of the Drosophila gene Serrate (Ser), which encodes an integral membrane protein. The extracellular domain contains two cysteine-rich regions, one of which is organized in a tandem array of 14 EGF-like repeats. Antibodies directed against part of the extracellular region confirm the localization of the protein in the membrane. In the wing imaginal discs, the protein is detected in those regions that are affected in the wings of two dominant mutations, SerD and SerBd. Both mutations as well as three out of eight newly induced revertants of SerD could be mapped molecularly to the transcribed region, confirming the identity between the gene Ser and the transcription unit characterized. During embryonic development, RNA and protein exhibit a complex expression pattern, which is, however, not correlated with an appropriate embryonic phenotype. Phenotypic interactions of Ser alleles with the neurogenic genes Notch and Delta coupled with the structural similarity of the proteins encoded by these three genes suggest close interactions at the protein level.