The aim of this study was to verify whether extremely low frequency (ELF) magnetic fields (MF) could affect transposition activity like some environmental stress factors such as heat shock or UV irradiation. Using an Escherichia coli LacZ strain transformed with a plasmid containing a Tn10 derivative element expressing -galactosidase only after transposition, it was possible to determine the events of transposition evaluating the rate at which the colonies developed dark coloured papillae (LacZ+). We found that those bacteria that had been exposed for a long time (58h) to a 50Hz low intensity MF (0.11mT) gave colonies with significantly lower transposition activity compared to sham-exposed bacteria. Such reduction in transposition activity was positively correlated to the intensity of the MF, in a dose-effect manner. This phenomenon was not affected by bacterial cell proliferation, since no significant differences were observed in number, diameter and perimeter between sham-exposed and MF-exposed colonies.