Molecular biological improvement of industrial solventogenic clostridia could be enhanced by a higher efficiency of electrotransformation. In this research, we used a new approach to determine the frequency spontaneously generated by Clostridium acetobutylicum ATCC 824 cells during the application of a square high-voltage pulse. Once the frequency of 100 kHz was determined we transformed clostridial cells with pSOS84 plasmid DNA using radio-frequency modulated high-voltage square pulses (electric field strength 12 kVcm(-1); pulse duration 22.5 ms; frequency of pulse modulation 100 kHz) to reach an efficiency exceeding 10(6) transformants mu g(-1) of plasmid DNA. We propose a possible role for cellular membrane structures in affecting the transformation yield.