The thermal and fluid effects caused by applied alternating electric fields during BiMn/Bi eutectic directional solidification is analyzed. The computational model for numerical analysis includes detailed analysis of the electrical and magnetic fields caused by the alternating current (AC) which results in an oscillating freezing rate pattern. This is presented, as well as the corresponding eutectic microstructure spacing. In this paper, numerical results demonstrated that in addition to process parameters, microstructure formation depends on the frequency of the applied AC, and it changes spontaneously as the alternating electric field is applied. (C) 2003 Elsevier B.V. All rights reserved.