The application of radio frequency (RF) electric fields as a nonthermal alternative to thermal inactivation of microorganisms in liquids was investigated. A novel RF system producing frequencies in the range of 20 to 60 kHz was developed. Electric field strengths of 20 and 30 kV/cm were applied to suspensions of Saccharomyces cerevisiae in water over a temperature range of 35 to 55 degrees C. The flow rate was 1.2 liters/min. The S. cerevisiae population was reduced by 2.1 +/- 0.1 log units following exposure to a 30-kV/cm field at 40 degrees C. The results of the present study provide the first evidence that strong RF electric fields inactivate microorganisms at moderately low temperatures. Increasing the field strength, the number of treatments, and the temperature enhanced inactivation. Frequency had no effect on inactivation over the range of frequencies studied.