Salmonella typhimurium LB5000 and Escherichia coli JM109 were transformed by electroporation. In accordance with the chemical transformation methods, the growth phase of these electrocompetent bacteria had a strong impact on transformation efficiency. Survival of bacteria after the high-voltage electrical pulse was also influenced by the growth phase. Both bacterial species were most successfully electrotransformed when microbial cells were harvested at the late lag phase. The second optimum for transformation reached E. coli cells in the mid-exponential and S. typhimurium cells in the late exponential phase. Transformation efficiencies ranged from 3.4 x 10(4) to 2.7 x 10(5) transformants per microgram DNA in the case of S. typhimurium and from 2.8 x 10(2) to 8.8 x 10(5) transformants per microgram DNA in the case of E. coli. Survival of cells after the electrical pulse in late lag and late exponential phases was about 20% higher than during other phases of growth. Preparing electrocompetent cells from later phases of their growth is more useful for practice, because it provides more biomass with good yield of transformants.