Strains of 5 cucurbit-infecting formae speciales of Fusarium oxysporum were collected from various locations in Japan. The assay for pathogenicity showed the complexity of their infection spectra. In some cases, a single forma specialis infected multiple genera or species of the family Cucurbitaceae, and different formae speciales shared the same host plants. In other cases, a single forma specialis contained subgroups, designated as races, differing in pathogenicity to a set of differential cultivars within the same plant species. Genetic relatedness among strains of the cucurbit-infecting F. oxysporum was determined by DNA fingerprinting with the fungal nuclear repetitive DNA sequences. Four repetitive DNA sequences used as probes were chosen from a genomic library of F. oxysporum f. sp. lagenariae and designated as FOLR clones. DNA fingerprinting of total DNA of strains from the cucurbit-infecting formae speciales after hybridization with FOLR sequences enables to distinguish strains not only at the forma specialis level but also at the race level. On the basis of DNA fingerprint profiles, phylogenetic relationships among the strains were analyzed by parsimony and UPGMA methods. The analyses indicated that the formae speciales and the races are intraspecific variants specialized in DNA level as well as host range, and that FOLR DNA fingerprinting can be used for the differentiation of these pathogenic veriants.