Over 100,000 prototrophic survivors from ultraviolet-irradiated stock were screened for pathogenicity on apple fruits. Of these, seven showed altered pathogenic capacity when compared with the wild type, five having reduced virulence and two being completely avirulent. The nutritional hypothesis of pathogenicity, as presently conceived, seems inadequate to explain the observed changes in virulence. It is suggested that, in the complex association of host and pathogen, the mechanism controlling pathogenicity must be considered as distinct from that which governs the basic metabolism of the parasite.