Some responses of the cellular slime mold Dictyostelium discoideum to ultraviolet light (UV) irradiation were investigated by analyzing two aspects of deoxyribonucleic acid (DNA) excision repair in the vegetative cells: (i) the fate of thymine-containing dimers and (ii) the production and rejoining of single-strand breaks. Experiments were done with the parental, radiation-resistant NC-4 strain and with the radiation-sensitive gammas-13 strain. The majority (greater than 85%) of the thymine-containing dimers produced in both strains by an energy fluence of 100/Jm2 were removed from the acid-insoluble DNA fraction within the first 3 to 4 h of reincubation in the dark. Moreover, as measured by alkaline sucrose gradients, single-strand breaks appeared in the DNA of both NC-4 and gammas-13 irradiated cells very rapidly and at low temperatures. This was presumed to be a result of the incision (nicking) step of excision repair as performed by UV-specific endonuclease(s). In NC-4 the time required for dimer excision correlated with the sealing of breaks as well as with the UV-induced division delays. In gammas-13 the single-strand breaks were closed at a slower rate than in NC-4. However, this was not accompanied by more extensive division delays.