ABSTRACT A population of Ralstonia solanacearum race 1 from tomato (Lycopersicon esculentum) was analyzed for genetic polymorphism and aggressiveness on tomato. The 46 strains were collected from main tomato-growing areas in Taiwan. Genetic analysis was achieved by two polymerase chain reaction (PCR)-based methods: REP-, ERIC-, and BOX-PCR (collectively as rep-PCR) and random amplified polymorphic DNA (RAPD) techniques. RAPD (with three 10-mers) and rep-PCR revealed 35 and 30 haplotypes, respectively, that were grouped in 14 clusters and 3 clusters, respectively. Distribution of strains into genetic clusters did not appear related to biovar or geographic origin in considering RAPD, rep-PCR, or composite data. Although strains were more dissimilar based on RAPD data than on rep-PCR data, the two techniques gave complementary results for strain clustering. A set of 40 strains representing the main haplotypes was inoculated on six tomato cultivars differing in their bacterial wilt resistance. Six groups differing in general level of aggressiveness and cultivar specificity were detected. Although populations were highly diverse in both genotype and aggressiveness, no association was found between the two characteristics. Although the sample sizes in this study were not adequate to draw definite conclusions about population structure, these results will be valuable for future population genetic studies on R. solanacearum.