A monospore analysis of fresh isolates of Verticillium albo-atrum Reinke & Berthold from eight locations and six varieties of potato in Prince Edward Island, showed that, with one exception, the mass isolates were composed of morphologic forms distinguishable in growth habit and in capacity to form dark resting mycelium. One chief component, resembling the parent form, was present in all isolates. Pathogenicity trials in the greenhouse showed that the monospore forms differed in virulence and that virulence was not associated with the capacity to form dark, resting mycelium. Some monospore forms were avirulent, and one series was found that was equally pathogenic on both the highly susceptible variety Irish Cobbler and the field resistant variety Houma.