From the standpoints of both culture and electronmicroscopy the author studied the destructive action of ultrasonic wave on various bacteria such as B. typbosa, B. coli, B. pyocyaneus, and Staph. aureus, and obtained the following results: 1. B. typhosa, B. pyocyaneus and Staph. aureus all possess about the same degree of resistance against ultrasonic wave, and they can be destroyed all with about the same strength of ultrasonic irradiation. B. coli, however, demonstrated a far greater resistance against ultrasonic irradiation. B. coli, however, demonstrated a far greater resistance against ultrasonic wave than any one of these bacteria. 2. The majority of B. typhosa, B. coli, and B. pyocyaneus, when cxposcd to ultrasonic irradiation, take such steps as firstly swelling of the cell, and then the destruction of the cell membrane either at extremities of longitudinal axis or at the point adjacent to the extremity and discharge of cytoplasm. 3. Most of Staph. aureus, when exposed to ultrasonic irradiation, are destroyed by taking such a course as first the uncoupling of staphylococcic chain, the swelling of those cell bodies thus broken free, and next a part of the swollen and dilatated cell membrane burst open and the outflow of cytoplasm.