Paramecium tetraurelia cells age with repeated cell cycles and finally die after a specific number of cell divisions. In addition to this clonal aging, starvation results in cultural aging in division-arrested cells. They are reset to time zero to initiate a new clonal life cycle, if autogamy occurs when starved. Increased UV sensitivity is one of the representative markers of clonal aging in P. tetraurelia. We studied changes in UV sensitivity with the advance of cell cycle, clonal age, and cultural age. In this study, we made the following observations: 1) cells of any clonal age became less sensitive to UV as the cell cycle advanced; 2) cells became more sensitive to UV as clonal age advanced; 3) cells became temporarily hypersensitive to UV through an unknown mechanism at the clonal ages of less than 80 fissions; 4) cells at the stationary phase were more sensitive to UV than those at the log-phase; and 5) both autogamy-mature and immature cells became more sensitive to UV as cultural age advanced and became less sensitive with similar kinetics when transferred to culture medium. The fact that UV sensitivity increased with cultural age irrespective of the occurrence of autogamy suggests that this is caused by a different mechanism from that associated with clonal age because the latter could be canceled by autogamy.