To clarify the diversity of blast (Pyricularia oryzae Cavara) races in West Africa, we investigated the pathogenicity of 96 blast isolates collected from different ecosystems in six countries, Bénin, Burkina Faso, Côte-d’Ivoire, Ghana, Mali, and Nigeria, and characterized using rice (Oryza sativa L.) differential varieties for 23 resistance genes and a susceptible control variety Lijangxintuanheigu (LTH). Virulent blast isolates occurred with high frequencies against LTH or differential varieties carrying Pia, Pik-s, Pi19(t), Pi12(t), Pit, Pii, Pi3 and Pi5(t). Conversely, they occurred at low frequencies against differential varieties carrying Pish, Pi9(t), Piz, Piz-5, Piz-t and Pita-2 and at intermediate frequencies against those carrying Pib, Pik-m, Pi1, Pik-h, Pik, Pik-p, Pita and Pi20(t). The isolates were recharacterized as reaction types of five groups, U, i, k, z and ta; accordingly, LTH and the differential varieties were categorized based on the chromosome locations of the resistance genes harbored in each genetic background. Twelve, seven, thirteen, eight and seventeen reaction types were found in groups U, i, k, z and ta, respectively. Thirteen of these, namely U43, U63, i7, k100, k106, k177, z00, z03, z04, ta003, ta031, ta403 and ta431, showed high frequencies of blast isolates and were considered dominant reaction types. We used the infection types of the differential varieties against these blast isolates to conduct a cluster analysis, and the isolates were classified into two clusters, I and II. Substantial differences in frequencies between both clusters were found in the reactions of differential varieties carrying these genes in the Pii, Pik and Pita chromosome regions. Both clusters I and II were distributed in an upland ecosystem with high and similar frequencies, whereas group II was mainly distributed in the irrigated lowland. These results suggested that many types of blast races were distributed in the upland ecosystem, while the limited ones were in lowland in West Africa. Finally, these blast isolates were categorized into 79 races in accordance with a new designation system based on the five differential variety groups used for reaction typing.