Ultraviolet (UV) inactivation of green colony-forming ability of several different types of Euglena gracilis var. bacillaris was studied. The observed target numbers are not widely different, while the doses required to produce a single inactivation event (D(o)) vary with the type of cell used. In dark-grown cells adapting to the light in resting medium and in an X-ray-induced mutant, D(o) is proportional to the chlorophyll content of the cells. However, in hyperdeveloped cells which contain abnormally high amounts of chlorophyll, the correlation does not hold, suggesting that it is not chlorophyll per se which is responsible for the differences observed. D(o)'s of colony-forming ability (viability) of light-grown and dark-grown cells are found to differ by the same factor as those of green colony-forming ability. Stationary phase and exponential phase cells show a small difference in D(o) with no obvious difference in target multiplicity. The multiplicity of the various target curves has been re-evaluated by computer and found to be between 30 and 40.