Uranium and thorium were found to be accumulated extracellularly on the surface of Aspergillus flavus. The rate and the extent of accumulation were subjected to variations in environmental parameters such as pH, temperature and the interference of certain anions and cations. The rates of uranium and thorium uptake by Aspergillus flavus were increased by chemical pretreatment of the cells. Accumulated uranium and thorium were removed chemically from Aspergillus flavus cells. The rate of uptake of uranium and thorium at 25 degrees C+/-3 degrees C was found to be extremely rapid at pH values of 4.5+/-0.2 and 2.5+/-0.2 respectively. Electron microscopic examinations showed that uranium and thorium were accumulated in a dense layer around the surface of Aspergillus flavus cells.