SUMMARYA study has been made of the factors controlling the uptake of captan from aqueous solution by conidia ofNeurospora crassa.The uptake process has been found to deviate from normal diffusion in the effects of captan concentration, temperature, salts, oxygen supply, and metabolic poisons. It is suggested that the captan absorbed by the spores is decomposed on or within the cell protoplast by a process requiring cell metabolic energy. Pre‐treatment of the spores with thiol reagents such as iodoacetic acid andp‐chloromercuribenzoate markedly decreased the uptake of captan without affecting spore viability or captan toxicity. This suggests that much of the high level of captan uptake required to inhibit spore germination is in fact being decomposed by cell thiols before reaching its site of toxic act