Bacterial wilt, caused by Ralstonia solancearum (Rs) (Smith) Yabuuchi et al., is one of the most important diseases of potato crops worldwide, including Madagascar. From the literature, strains of Rs phylolypes III or 1 were pathogenic to Solanaceae in the highlands potato areas (1,2). From years these strains were managed through the deployment of resistant or tolerant potato cultivars thanks to joint 30-yeaTS of collaborative research undertaken by Fifamanor (3) and CIP (4). however, since the last five years severe epidemics of bacterial will were observed in the Vakinankaratra highlands. A quest ion arises about the durability of resistant or tolerant potato varieties. It was hypothesized that (i) highly aggressive strains may have emerged from the Rs populations or ( ii ) exotic strains could have been introduced. A large survey was organized to collect strains of Rs to investigate how diverse the actual Rs population was. A first approach focused on phylotype profiling (multiplex-PCR) and phylogenetic assignation (egl sequcncing) of a limited number of strains (n=32) isolated from highly diseased potato plots. Surprisingly, this first set of Rs strains were computed phylotype IIB - I based on egl-treeing including worldwide reference sequences that covered the known diversity. This is the first report of brown rot causing strains in Madagascar. From the survey, 763 strains have been collected from symptomatic plants in main representative potato growing areas of Vakinankaratra: 132 strains were phylotype III and 631 phylotype II B-I. Phylotype III strains were associated to sites with low bacterial will incidence. In contrast., phylotype IIB-I strains were always associated with severe outbreaks and found to be distributed over almost the entire area surveyed. Population structure and evolutionary dynamics o f lineages IIB-I and III are currently investigated by improving available MLVA (multilocus VNTR analysis) schemes (5.6) and by developing a specific MLVA scheme, respectively. (Résumé d' auteur)