Knowledge of the avirulence (Avr) genes present in plant pathogens facilitates the identification of the required resistance (R) gene to be incorporated in host plants to overcome pathogen infection. The plus/minus Polymerase Chain Reaction (PCR) assay was conducted to determine the presence of the virulent and avirulent ACE1 genotypes of the rice blast fungus in Philippine rice fields. Results indicated that the avirulent genotype Guy11 was present only in 13% of the test isolates, whereas the virulent genotypes, PH14 and CM28, appeared in 83% of the total number of test isolates. With contingency analysis on the type of agroecosystem and presence of ACE1 genotype, it was known that Guy11 is significantly associated with the upland agroecosystem. Whereas, PH14 was present in both the irrigated and rainfed lowland and CM28 was ubiquitously associated with any type of agroecosystems. No significant association, however, was noted between a specific ACE1 genotype and geographic location. The isolates having virulent ACE1 genotype can overcome the resistance Pi33 which is commonly introgressed into popular semi-dwarf indica rice varieties.