When sporulating cells are treated with the organic solvents (xylene, toluene, octanol and chloroform), with TCA or with moist heat (10 min at 80 degree C), the sporangial cells exhibit the same ultrastructural changes as do the vegetative cells. The forespores undergo similar changes after early but not after late treatment. Resistance toward the killing effect and toward ultrastructural alterations appear in the same order. One could correlate the appearance of resistances with precise ultrastructural events as follows:--xylene resistance: cortex formation (stage IV);--resistance to toluene, octanol and chloroform: coat development and cortex maturation (respectively early, middle and late stage V);--TCA and heat resistance: spore maturation (stage VI). The possible mechanisms of the chemical resistances are discussed.