The application of the residual growth technique on strain lys-1 of Salmonella typhimurium reveals delayed appearance of UV-induced mutants to wildtype. After irradiation (300 or 450 ergs/mm2) the increase of newly arising reversions takes place over the whole range of 16 generations covered by the technique used. On enriched minimal medium the irradiated but nonmutated cells proved to have a lag phase of 3 hours. The respreading method of Newcombe between 6½ and 7 ½ hours after irradiation shows doubling in number of cells being potent to give rise to wildtype colonies. Later for at least 3 h this number remains constant. Penicillin from 0-6 h and 3-6 h after irradiation kills lys- and potentially reverted cells exactly to the same extent. Therefore during the period of nonincrease of potential mutants these cells belong to normally growing clones. However, the potentiality to express the mutant character, is passed only to one out of two daughter cells, which arise from the division of a potentially mutant cell. Different possible mechanisms for this mutation delay are discussed.