The nucleation rate of tetragonal hen egg white (HEW) lysozyme crystals could be increased or decreased via application of an ac electric field, which modifies the chemical potential of the liquid and solid states. The strength of an external electric field necessary to control the nucleation rate was calculated to be 104-105 V/cm, which was larger than that used experimentally by 2 or 3 orders of magnitude. This huge discrepancy could be understood by assuming the presence of a very large electric field sustained in the electric double layer (EDL) that forms at the interface between the dissimilar phases. By changing the frequency of an applied electric field of 800 V/cm in the range of 1-9 MHz, we were able to increase or decrease the nucleation rate. Specifically, the nucleation rate with an applied electric field increased with an increasing applied frequency up to 5 MHz and then significantly dropped above this value. The rapid decrease is considered to result from extinction of the EDL above 5 MHz.