The radiotoxic spectral sensitivity of the unicellular green alga Chlorella vulgaris has been determined for 20 wave lengths in the ultraviolet ranging from 2250 A to 3022 A. As a preliminary step to facilitate deductions, an experiment was performed with three relative intensities which demonstrated that for a range of 12-fold the lethal effect as measured by the reciprocal of the reaction time is approximately proportional to the product of the exposure time and intensity. The results are in good accord with earlier observations as far as those went. They show maximum lethal sensitivity at about 2600 A. The wave length 3130 A, slightly longer than the short wave length limit of solar radiation reaching the earth's surface, had no lethal effect on the algal cells although of a greatly higher intensity than the toxic wave lengths. Since it is true that death ensued more quickly in some regions than in others, the radiotoxic virulence, or speed of effectiveness of each lethal ray in killing the algal cells for a radiotoxic quotum has been calculated for 20 wave lengths ranging from 2250 A to 3022 A. The results are of a small percentage of probable error, and they show an interesting wave of virulence alternately waxing and waning with decreasing wave length, but reaching a high maximum at 2323 A.