The variation of microorganisms (activated sludge, Saccharomyces cerevisiae, Aureobasidium pullulans) caused by the biosorption of Pb2+ was observed by TEM and microscope. By the TEM observation of S. cerevisiae, the plasmolysis and lysis of cell wall or cell membrane were occurred by the penetration of Pb2+ into the inner cellular region. However, in the case of A pullulans, the plasmolysis and lysis of cell wall or cell membrane were not occurred because of the prevention of Pb2+ penetration by the extracelluar polymeric substances (EPS). A flocculation of microorganisms, in the case of A. pullulans, was observed by the Pb2+ accumulation after 3∼4 h and the color was changed from white to black after 1 day. The flocculation of activated sludge was improved by the accumulation Pb2+ after 1 h, however, the floc was broken up and the settling efficiency decreased after 1 day.