In a study of the validity of the Bunsen-Roscoe law for the biological action of x-rays, freshly laid eggs of Drosophila were irradiated with x-ray dosage, between 83 r and 220 r delivered in times between 3600 secs. and 2 secs. The subsequent death rate was dependent only on the total r units delivered for all intensities greater than 5 r per minute below this limit the death rate diminished rapidly. Some evidence was found of a slight periodic fluctuation of sensitivity to radiation with time.