Summary:Seven species of bacteria, including both rods and cocci, were exposed in dilute suspension in solutions of dried skim milk and in nutrient broth to ultrasonic vibrations of 1 Mc./s. frequency.Bacterium coli, Lactobacillus caseiand aMicrobacteriumspecies were killed fairly rapidly,Bact. colibeing the most sensitive.Staphylococcus aureus, Streptococcus lactis, Strep, faecalisandStrep, pyogenesshowed at first a rise in the viable count, followed by a decline in numbers. The cocci were all more resistant than the rod‐shaped species tested. Similar tests at 20.5 Kc./s., but with much less power absorbed, showed a slower decline in numbers withBact. colithan at 1 Mc./s. There was no decrease withStaph, aureus, but there was more rapid death ofStrep, lactisduring the second half of the treatment period. In relation to power absorbed, the lower frequency was the more effective.Under conditions which caused rapid death ofBact. colisuspended in broth, complete protection was afforded to that organism by fresh whole milk: smaller degrees of protection were seen in both homogenized and separated milk. Protection was also conferred by dried skim milk, casein, gelatine and peptone in solution, the effect increasing with concentration. High concentrations of lactose and gum tragacanth also had some effect.WhenBact. coliwas treated at 1 Mc./s. in broth, there was no lethal action unless the power absorbed exceeded a quite large threshold value: no threshold was detected at the lower frequency.The death rate ofBact. colitreated at 1 Mc./s. in various liquids was as a rule not constant: it usually diminished with time. The shape of the survivor curve was greatly altered when the volume (depth) of suspension treated was increase