A comparison was made of growth factor requirements and growth factor synthesizing capacity of bacteria from the root surface and from the seeds of flax varieties resistant and susceptible, respectively, to wilt caused byFusarium oxysporumf.lini. The percentage of bacteria from both root and seed which required growth-promoting substances was greater with the resistant ('Bison') than with the susceptible ('Novelty') variety. On the other hand bacteria capable of synthesizing growth factors were proportionately more abundant with the susceptible variety. The findings indicate a more pronounced rhizosphere effect exerted by the susceptible than by the resistant variety, and point to circumstances that favor a greater accumulation (or 'turnover') of vitamins and related substances at the root surface of susceptible plants.Two strains ofFusarium oxysporumf.linicapable of inducing wilt required no vitamins for growth; thiamine had a depressing effect. Both strains were able to synthesize various growth factors, though in different degree. Culture filtrates of bacteria from roots of the two varieties of flax showed no difference in their effect on growth ofFusarium; however, bacteria from the resistant variety showed somewhat greater ability to depressFusariumthan those from the susceptible. Though the findings do not constitute evidence of any relationship between differences in growth factor relationships to pathogenic potential ofFusarium, they suggest the value of considering growth-promoting substances in studies of the effect of nutrition on the activity of soil-borne pathogens.