Spores and vegetative forms of a strain of B. megatherium were irradiated by ultraviolet light of the wave lengths 2536 A, 2803 A, and 3132 A. The killing rate of both bacteria and spores is exponential, in agreement with irradiation results on other bacteria. Twice as much incident energy is needed to kill the spores as the vegetative forms (50 per cent death). The absorbed energy per bacterium for 50 per cent killing has been calculated on the assumption that the absorption of the vegetative cells is the same as that of colon bacilli. These results are compared with previous measurements on other bacteria.