Exponentially growing cells of S. cerevisiae were cultivated in the concomitant presence of 2×10^(-3) M paraquat for 12 hours. Cellular growth became slower as the time passed and then, it was completely inhibited after g hours of cultivation. More than 70 percent of the yeast cells were killed with in 12 hours of paraquat treatment. It was observed that size of the yeast cell varied from 4.5×5.5㎛ to 2.5×3.5㎛ in diameter, while that of control was uniform. The number of budding cells became rare, Cell wall of the paraquat treated cell was thinner(0.15㎛) than that of control cell(0.20㎛). Outer part of the cell wall had higher density than inner part. Membrane structures such as plasma membrane and mitochndria was decomposed during the paraquat treatment for 12 hours.