We report on the effect of ethanol and acetaldehyde on yeast chromosomal DNA and on isolated DNA. Ethanol induced DNA single-strand breaks in repair deficient but not in repair proficient Saccharomyces cerevisiae. Acetaldehyde has a deleterious effect on chromosomal DNA in cells as well as on isolated DNA. The results presented support earlier data to show that ethanol is mutagenic via its first metabolite, acetaldehyde.