The variation in two aspects of pathogenicity, aggressiveness and virulence (physiologic race), among single zoospore cultures from three wild isolates has been examined. Two components of aggressiveness were measured, rate of growth on tubers and generation time on detached leaflets. Virulence was assessed from the pattern of compatible and incompatible reactions to members of the R gene differential series. Extensive variation in aggressiveness was detected in samples of zoospore cultures from all three isolates. The level of aggressiveness ranged from a high, equivalent to that of the parental wild isolates, to a complete lack of pathogenicity; as much as 45% was of the latter type. No instance of a change in virulence was found among 104 cultures tested.The significance of the observed pathogenic variation is discussed. It is concluded that zoospore variation is not important as a source of new physiologic races or strains adapted to particular, horizontally resistant varieties. Comparison of populations of zoospore cultures with samples of wild isolates suggests that directional selection for high growth rate and high and rapid sporulation operates in nature. The correlations between a number of cultural and pathogenic characters are examined, and an association between abundant sporulation in culture and high aggressiveness demonstrated.