Eight blast resistance genes --Pia, Pib, Pik-s, Pik-h, Pita, Piz-t, Pi20, and one of three (Pii, Pi3, or Pi5(t)) were identified in 42 IRRI-bred varieties following a differential system using Philippine blast isolates. Among these genes, two genes Pib and one of Pik alleles (Pik-s or Pik-h), were found in almost all IRRI varieties. The number of resistance genes in each variety varied from two to five, and the kinds of gene combination were also limited. This limitation might be due to the gene sources such that IR8, IR24, IR36, or their hybrid progenies were often included in the breeding of these varieties. Using the genefor-gene theory, this study was carried out in three steps: 1) estimation of genes following the reaction patterns of monogenic lines to blast isolates, 2) genetic analysis using BC1F2 populations with a susceptible variety as recurrent parent, and 3) allelism test with Japanese differential varieties. Through genetic analysis, each investigation’s ability to identify resistance genes was demonstrated.