Three sesquiterpene lactones (hymenoxon, helenalin, and tenulin) were tested for genotoxicity using six strains of Bacillus subtilis. Hymenoxon was found to produce lethal DNA damage in strains rec A8 and mc-1 while helenalin was lethal in strains mc-1 and rec E4. Tenulin did not produce lethal DNA damage in any of the strains tested.