Following treatment of the thermotolerant ethanol producing yeast strain Kluyveromyces marxianus IMB3 with single electric field pulses of 2.4kV/cm, ethanol production was found to increase from 25 to 35% of the theoretical maximum yield within 20 hours, during growth on lactose-containing media (4% [w/v]). Following treatment with electric fields and addition of 1mM MnCl2 to lactose-containing media, ethanol production almost doubled within a 30 hour period. In addition, low levels of functional beta-galactosidase activity could be detected in the extracellular culture filtrates, presumably as a result of electropermeabilization.