Using Chlorella ellipsoidea as material, investigations were made of the effects of ultraviolet irradiation upon various activities of cells at different developmental stages in their life cycle. Cell activities investigated were photosynthesis, respira tion, over-all growth, modes of synchronous growth and cell division as well as the formation of nucleic acids. The u. v.- light applied was 30 μμW/cm2in intensity and 2537 Å in wavelength. The most u. v.-sensitive was the over-all growth activity, and in this respect the irradiation applied at the L2-stage was more inhibitive than that given at the D-stage. The next most vulnerable was the photosynthetic activity, the sensitivity being the same in the D- and L-cells. The most resistant toward u.v. was the endogenous respiration of D-cells followed by their respiration using exogenous glucose as substrate. The L2-cells appeared to be unable to use exogenous glucose as substrate of respiration, but their endogenous respiration was considerably stronger than that of D-cells, and its u. v.-sensitivity was the same as that of glucose respiration of D-cells. When D-cells were u. v. irradiated immediately before the start of synchronous culture, growth and cell division as well as the formation of DNA and RNA were retarded in proportion to the u. v.-dose applied. The division number (n) was normal (around 4) at lower u.v.-doses (1-2 minute irradiation), but was reduced to a half (about 2) at a higher dose. When, during the synchronous culture, 1-minute u.v.- irradiation was applied at various stages of the ripening phase, the division was retarded, but the cells, after attaining an abnormally large size, divided into about 8. If the irradiation was given at the L4-stage, the division number was practically unmodified (n=4.5), although the division was somewhat retarded compared with that of the control culture. When a 1-minute irradiation was given at the L2-stage, there occurred an apparent stimulation of DNA- and RNA-formation, a phenomenon which corresponds to the production of a larger number of daughter cells than it was the case in control cultures. Thus the cells which were moderately u.v.-irradiated at different stages of synchronous culture were able to complete their life cycle, but later a certain portion of irradiated cells became unable to grow normally.