The microstructure of periodic fluctuant dendritic theta in a matrix of alpha + theta eutectic was obtained in an Al-38 wt% Cu alloy processed by ACRT-B method grown at growth velocities (V) ranging from 5 to 60 mu m/s and crucible rotating in a trapezoidal way with maximum rotation rates (Omega(max)) ranging from 100 to 400 rpm. Formation of this structure is explained by the influence of the periodical Ekman flow on the growth of dendritic theta during the spin-up and spin-down process. It was also observed that the alpha + theta eutectic between the primary dendrites of theta (CuAl2) is not periodic and fluctuant during ACRT-B process. This is quite different from our previous observation of periodic eutectic in Al-Cu eutectic processed by similar ACRT-B method.