The microsclerotia of V. albo-atrum are composed of cells varying in size, shape, cell-wall thickness and pigmentation. Only the thin-walled, less pigmented cells germinated. The fungistatic principle found in all natural soils inhibited germination of both conidia and microsclerotia of V. albo-atrum. The fungistatic effect was overcome in the presence of the roots or root exudates of tomato (a host) and wheat (nonhost). The increase in germination of microsclerotia was considerably greater with tomato, however, Frac-tionation of tomato-root exudate demonstrated that the stimulatory effect of the root exudate in overcoming fungistasis was in the basic fraction (mainly amino acids) rather than the acid- neutral fraction (organic acids, phosphate esters, nucleotides and sugars).