The evaluation of a total of 122 blast (Pyricularia oryzae Cavara) isolates collected from the Tonle Sap and Mekong river regions of Cambodia revealed wide variation. Using a new designation system, the blast isolates were categorized into 92 races based on the reaction patterns of rice (Oryza sativa L.) differential varieties (DVs) harboring 23 resistance genes and of 1 susceptible cultivar, Lijiangxintuanheigu (LTH). Cluster analysis was used to classify the blast isolates into 3 groups -I, IIa, and IIb -using data from these reaction patterns of the DVs and LTH. We used the classifications established under the new designation system, alongside cluster analysis and the geographical distribution of blast isolates, to investigate the diversity and differentiation of blast races in the Tonle Sap and Mekong river regions. The distributions of the blast races differed between the 2 regions, although blast isolates of group IIa were distributed commonly in both regions and groups I and IIb occurred at higher frequencies in the Tonle Sap region rather than the Mekong region. The blast isolates in groups I and IIb were also less diverse than those in group IIa. Accordingly, Group II blast isolates overall were distributed in both regions with high diversity, but some modified blast isolates were additionally distributed in the Tonle Sap region. We also investigated the pathogenicities of blast isolates from wild rice (Oryza rufipogon Griff) weeds neighboring the cultivated rice, and discuss the relationship between these isolates and those from cultivated rice.