1. Blepharisma undulans, a protozoan with a reddish pigment, shows increased oxygen consumption under the influence of light.
2. If the light intensity is high, the animals are killed during a burst of oxygen consumption.
3. If the blepharismas are first bleached by exposure to light of low intensity they show only slightly increased oxygen consumption under the influence of light and they are not killed.
4. A preparation in which the animals are killed by heat still shows the increase in oxygen consumption on illumination with brilliant light. The supernatant solution does so as well, as does an alcohol extract of the dye.
5. The conclusion is drawn that the blepharismas are killed during photooxidation of the pigment, but the mechanism of action is not clear. Several possibilities are considered in the discussion.