Analysis of sedimentation profiles in alkaline sucrose gradients showed that, through a metabolic process, formaldehyde (FA) produced single-strand breaks in DNA of exponential phase cells of haploid wild-type Saccharomyces cerevisiae. The production of this type of lesion was dose-dependent. Strains defective in excision-repair of pyrimidine dimers induced by ultraviolet (UV) irradiation showed a reduced capacity to undergo single-stand breaks after treatment with FA. This indicates that the repair pathways of damage induced by UV and FA share a common step. Post-treatment incubation of wild-type cells in growth medium indicate a lag in cell division during which a slow recovery of DNA with a normal size was observed.