SUMMARYUnusual swollen and branched cell forms were produced in a thymineless mutant of Escherichia coZi strain c by treatment with mitomycin C, and to lesser extents, by incubation without thymine and by exposure to ultraviolet light. In the case of mitomycin C treatment the morphological changes were accompanied at first by degradation of deoxyribonucleic acid (DNA), later by degradation of ribonucleic acid (RNA) and inhibition of protein synthesis, and throughout by inhibition of DNA synthesis and extensive loss in viability. Thymineless incubation prevented DNA synthesis and also resulted in extensive killing. Cultures incubated after exposure to ultraviolet light exhibited a small amount of DNA degradation and a lag in DNA synthesis. Upon prolonged incubation with mitomycin C or without thymine many of the abnormal forms became very enlarged and eventually lysed. No evidence of bacteriophage or bacteriocin could be detected in the treated cells.