Due to increased public concern on the possible human effects of exposure to very low frequency magnetic fields (VLFMF) produced by household appliances , this paper examines the possible effects of VLFMF on the mutation of DNA molecules which is known to be a direct cause of cancer development. In this study, the bacterium Salmonella typhimurium TA1535/(pSK1002) was exposed to VLFMF and the expression intensity of the activity of β-galactosidase, which is located downstream of the umu SOS gene operon, was examined using umu assay method with a non-exposure control. The results of the umu assay of the damage to DNA molecules caused by VLFMF exposure are reported. t-test results on the sample data showed no significant difference in three out of the four sets of measurements acquired. These results show that the possibility of a weak relation between exposure to VLFMF and damage to DNA can not be denied. However, the reference level according to the umu assay method stays around the double and thus it can be concluded that no effects from exposure to 20 kHz, 600 μT magnetic fields in terms of damage in DNA molecules were observed.