A suitable plane transmission line was developed and its behaviour analysed at 900 MHz radiofrequency fields to study the DNA mutability and repair of microorganisms. In this work, utilizing such a device, we investigated the behaviour of the DNA mutability and repair of Escherichia coli strains. The transmission line was very simple and versatile in changing its characteristic resistance and field intensity by varying its sizes. In absence of cell samples inside the transmission line, the relative modulation of the electric and/or magnetic field was±31% with respect to the mean values, allowing the processing of more samples at different exposure fields in a single run. Slight decrease in spontaneous mutability to rifampicin-resistance of the E. Coli JC411 strain, was demonstrated in mismatch-repair proficient samples exposed to the radio-frequency fields during their growth on solid medium.