Spiral waves of intracellular Ca²⁺ have often been observed in Xenopus oocytes. Such waves can be accounted for by most realistic models for Ca²⁺ oscillations taking diffusion of cytosolic Ca²⁺ into account, but their initiation requires rather demanding and unphysiological initial conditions. Here, it is shown by means of numerical simulations that these spiral Ca²⁺ waves naturally arise if the cytoplasm is assumed to be heterogeneous both at the level of the synthesis and metabolism of D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] and at the level of the distribution of the Ins(1,4,5)P₃ receptors. In such conditions, a spiral can be initiated in the simulations after an increase in Ins(1,4,5)P₃ concentration, with the direction of rotation being determined by the position of the region of high receptor density with respect to the locus of Ins(1,4,5)P₃ production.