The radiotoxic spectral sensitivity has been determined for eight wave lengths in the ultraviolet ranging from 2652 to 3022 A as applied to a unicellular green alga Chlorella vulgaris. Although all the rays from 2652 to 3022 A killed the algae eventually, death ensued much more quickly in some of the regions than in others. The radiotoxic virulence or speed of effectiveness of each lethal ray in killing the algal cells for a radiotoxic quotum at eight wave lengths ranging from 2652 to 3022 A has been calculated.