The objectives of this study were to examine genetic variation, physiological dissimilarities and diversity in pathogenicity between Verticillium fungicola var. aleophilum and var. fungicola and within a population of six V. fungicola var. fungicola strains responsible for dry bubble outbreaks on mushroom farms. Genetic variability was investigated using random amplification of polymorphic DNA (RAPD). Mycelial growth rate, extracellular enzyme production and susceptibility to hydrogen peroxide were used to examine physiological dissimilarities. Variation in pathogenicity was studied both in vitro and during mushroom cultivation. All the physiological properties studied indicated that var. aleophilum isolates were potentially more efficient than var. fungicola isolates for rapid colonisation of the mushroom cultivation medium. They could then interact more efficiently with Agaricus bisporus to produce dry bubble disease. RAPD analysis confirmed that all the French isolates belonged to var. fungicola, and two isolates were distinguishable from the homogeneous group constituted by the others. These isolates had a higher mycelial growth rate and lower extracellular enzyme activities in liquid media, except for chitinases. Their spores were more susceptible to germination inhibition by hydrogen peroxide, and they were responsible for higher levels of affected mushrooms. The two varieties might be regarded as pathotypes that are geographically isolated, and variation in isolates of var. fungicola might have consequence for mushroom growers.