Two hypotheses which might explain a recent increase in the incidence of verticillium wilt of chrysanthemums in glasshouses in the Netherlands were investigated, viz whether selection for increased resistance to elevated temperatures has occurred due to frequent steaming of soils in the glasshouses, or whether the strains of Verticillium dahliae occurring in chrysanthemum glasshouses are particularly virulent towards this host. Following artificial inoculation, five isolates of V. dahliae from chrysanthemum were pathogenic on chrysanthemum but five isolates from potato were non-pathogenic for this host. When inoculated onto potato plants, all isolates caused early senescence with no significant difference between the two groups of isolates. In amplified fragment length polymorphism analysis, the potato isolates formed a cluster distinct from all other isolates. As a group the chrysanthemum isolates were no more diverse than the potato isolates but did not form a cluster distinct from 12 other isolates tested. This suggests that high pathogenicity to chrysanthemum has developed on several occasions but that the group of potato isolates were possibly monophyletic. Microsclerotia produced in vitro from the chrysanthemum isolates had significantly lower average lethal temperature tolerance than those from the five potato isolates suggesting that being able to resist the effects of soil sterilisation by steam is not a factor in wilt of chrysanthemums in the Netherlands.