A total of 94 blast isolates were collected from five provinces in the Mekong River Delta in southern Vietnam. The pathogenicities of these isolates were evaluated using 25 international differential varieties (DVs) covering 23 resistance genes and a susceptible Chinese cultivar, Lijiangxintuanheigu (LTH). Based on the reaction patterns of the DVs, the isolates were classified into two clusters (I and II). Isolates virulent towards the DVs for Pit, Piz-t, Pi19(t), and Pita were more frequent in cluster I than in cluster II. Isolates virulent towards the DVs for Pi3, Pi5(t), and Pita-2 were more frequent in cluster II than in cluster I. Differences were also found in their geographical distributions, with isolates of cluster I dominant in the southern region of the Mekong River Delta and those of cluster II dominant in the northern region. Finally, the blast isolates were classified into 67 races. Based on these results and information on donor rice cultivars, the relationships between blast races and cultivated rice varieties were discussed. This information will be useful for understanding the differentiation of blast races distributed along the Mekong River in the countries of Southeast Asia.