1. For the two motile bacilli, B. coli and B. aertryke, the absorption of a single 155 K.V. electron is sufficient to cause death. Furthermore, all, or nearly all, the electrons absorbed are lethal. The same is undoubtedly true of Staphylococcus aureus. In addition to providing a quantitative picture of the interaction of bacteria and cathode rays, these results suggest that radiation of the energy content used in our experiments is not suitable for altering the inheritable characteristics of bacteria.