The tomato Cf-9 gene confers resistance to infection by races of the fungus Cladosporium fulvum that carry the avirulence gene Avr9. The Cf-9 gene was isolated by transposon tagging with the maize transposable element Dissociation. The DNA sequence of Cf-9 encodes a putative membrane-anchored extracytoplasmic glycoprotein. The predicted protein shows homology to the receptor domain of several receptor-like protein kinases in Arabidopsis, to antifungal polygalacturonase-inhibiting proteins in plants, and to other members of the leucine-rich repeat family of proteins. This structure is consistent with that of a receptor that could bind Avr9 peptide and activate plant defense.