Technical Abstract: The rice blast system has long been recognized as a classical gene-for-gene system as described by Flor (Flor 1971; Kiyosawa 1971). Discovery of the sexual state of the rice blast pathogen allowed genetic confirmation that single avirulence (AVR-) genes in the pathogen corresponded to R-genes in rice (Silue et al. 1992 and references therein) as predicted by the gene- for-gene hypothesis. Genetic analysis in our laboratory also identified AVR-genes in the blast fungus (Valent et al. 1991; Kang et al. 1995; Sweigard et al. 1995), including one that determined avirulence toward Yashiro-mochi, a differential rice variety that contains the Pi-ta resistance gene (Yamada et al 1976). This AVR-gene, originally named AVR2- YAMO for rice variety Yashiro-mochi, was renamed AVR-Pita after subsequent analysis demonstrated that this AVR-gene did in fact correspond to Pi-ta and not to a previously unidentified R-gene in Yashiro-mochi 9K.S. Wu and B. Valent, unpublished results). We used map-based cloning approaches to clone both AVR-Pita from the pathogen and Pi-ta from rice. AVR-Pita appears to encode a neutral zinc metallopeptidase. Pi-ta encodes a predicted cytoplasmic protein of 928 amino acids containing a nucleotide binding site and a leucine-rice carboxyl-terminus. Our current hypothesis is that the Pi-ta protein is an intracellular receptor that binds to the AVR-Pita mature protease inside the host cell, initiating Pi-ta-mediated defense responses.