A total of 119 blast (Magnaporthe grisea (Hebert) Barr.) isolates collected from the Philippines, were characterized for their pathogenicities using 19 rice (Oryza sativa L.) differential varieties (DVs) targeting for 18 resistance genes. These isolates were classified into 31 groups based on the reaction patterns to 9 DVs for targeting 9 resistance genes, Pia, Pib, Pii, Pit, Pita, Pish, Piz-t, Pi3, and Piz-5, at first, and then further divided into 70 pathotypes considering the reaction pattern of the other 9 DVs for the other resistance genes, Pik, Pik-h, Pik-m, Pik-p, Pik-s, Pita-2, Piz, Pi1, and Pi20(t). Twenty isolates that have differentiating ability, stable reactions and good sporulating ability were selected. The reactions of these isolates were confirmed using the monogenic lines as a set of DVs for targeting 24 resistance genes, Pia, Pib, Pii, Pik, Pik-h, Pik-m, Pik-p,Pik-s, Pish, Pit, Pita, Pita-2, Piz, Piz-t, Piz-5, Pi1, Pi3, Pi5(t), Pi7(t), Pi9(t), Pi11(t), Pi12(t), Pi19(t), and Pi20(t). Several resistance alleles of Pik locus, Pik, Pik-h, Pik-m, and Pik-p, and Pi1 had the same reaction patterns and could not be differentiated by these selected blast isolates. No avirulent isolate for Pit and Pi19 was found. The three genes, Pish, Piz-5 and Piz, showed a wide spectrum of moderate resistances to these isolates. These findings suggested the existence of a wide variation of blast pathogens in the Philippines. This information on pathogenicity of blast isolates from the Philippines will be also useful to understand the differentiation and relationship between blast races and resistance genes. The monogenic lines as the DVs and the selected 20 blast isolates can be used as the first differential system, which can characterize the resistances of rice varieties and pathogenicities of blast isolates in the tropics.