Regeneration of postperistomial pieces of Blepharisma intermedium was retarded by increasing doses of UV 265 nm; a dose of 3 200 ergs/mm −2 usually doubled the time for 50 % regeneration while one greater than 5 600 ergs/mm −2 generally stopped regeneration. However, a dose of 4 000 ergs/mm −2 just detectably reduced incorporation of 14C-uridine into RNA (RNAase-removable) in and near the macronucleus, doses of 4 400 ergs/mm −2 and more produced progressively greater effects, reduction of tracer incorporation being evident both in degree of labeling of individual cells and number of cells strongly labeled. Incorporation of 14C-thymidine into DNA (DNAase-removable) in and around the macronucleus of those postperistomial pieces which showed any incorporation of Blepharisma was reduced by UV doses of 4 000–5 000 ergs mm −2, as was incorporation of 14C-amino acids into proteins. Growth in bromodeoxyuridine did not sensitize fragments of Blepharisma to UV. Regeneration was retarded most by UV in cells irradiated immediately after cutting. However, incorporation of 14C-uridine into postperistomial pieces continued throughout regeneration and was about equally sensitive to UV 4 h after cutting as immediately afterwards. Regeneration was also retarded more if irradiated whole cells were cut soon after exposure than if cut later. However, incorporation of 14C-uridine continued throughout regeneration and was about as sensitive to UV in those cut later as in those cut earlier. The macronuclear condensation cycle which accompanies regeneration was retarded by UV, the delay increasing with dose.