1. The nature of ultraviolet injury and its variation with the same dose given at different intensities and wave lengths have been investigated in the protozoan Didinium nasutum , using time to the fourth division as a measure of injury. 2. The injury has been found to consist of a "slowdown" of division rate, which always occurs, and a "stasis," usually at the second division after irradiation, which appears in varying degrees among more severely injured samples. 3. Injury was found to be almost independent of intensity at three wave lengths out of four studied over a wide range of intermediate and high intensities, but was found to rise sharply with lower intensity at all except the longest wave length. 4. Flashed UV of high intensity is much more effective than the same dose of continuous radiation at high intensity and shorter total time of treatment. It is also more effective than the same dose at low intensity and equal time of treatment, though only slightly so. 5. An increase of injury with rise of temperature and with increase of dark period clearly indicates that injury depends on thermochemical reactions following the absorption of UV in Didinium . 6. The most reasonable assumption is that a similar conclusion applies to other organisms as well, and that its general application may be useful in the investigation of UV effects on protoplasm.