Aggressiveness of conidia ofBipolaris sorokiniana(Sacc.) Shoemaker on wheat seedlings was reduced after 30 days of incubation on sterilized soil inoculated with microorganisms. The effect was greater with bacteria than with fungi or actinomycetes. Recovery of aggressiveness occurred when stressed conidia were incubated with nutrients. With increasing stress a longer period of incubation was needed for recovery. The14C exudation from conidia was greater in unsterilized soil than in soil containing specific microorganisms. Exudation from conidia was inversely related to aggressiveness and germination.