Genetic mapping showed that the rice blast avirulence gene AVR-Pita is tightly linked to a telomere on chromosome 3 in the plant pathogenic fungus Magnaporthe grisea. AVR-Pita corresponds in gene-for-gene fashion to the disease resistance (R) gene Pi-ta. Analysis of spontaneous avr-pita(-) mutants indicated that the gene is located in a telomeric 6.5-kb BglII restriction fragment. Cloning and DNA sequencing led to the identification of a candidate gene with features typical of metalloproteases. This gene is located entirely within the most distal 1.5 kb of the chromosome. When introduced into virulent rice pathogens, the cloned gene specifically confers avirulence toward rice cultivars that contain Pi-ta. Frequent spontaneous loss of AVR-Pita appears to be the result of its telomeric location. Diverse mutations in AVR-Pita, including point mutations, insertions, and deletions, permit the fungus to avoid triggering resistance responses mediated by Pi-ta. A point mutation in the protease consensus sequence abolishes the AVR-Pita avirulence function.