Antibacterial effects of the water-soluble extract of Ulmus pumila L. on Staphylococcus aureus and Escherichia coli were studied by cylinder plate dilution method. The cells of S. aureus and E. coli treated with the extract were comparatively examined by scanning and transmission electron microscopies for their morphological features. The normal cells of S. aureus and E. coli showed thypical coccus and rod shapes, respectively, but the cells of S. aureus treated with the extract showed rough surface structures with many granular protrusions and were destroyed to form ghost cells by liberating their cytoplasmic components. E. coli cells treated with the extract were destoryed without enlarging in size and producing granules on their surfaces.