Macropore formation during anodization of highly resistive p-type crystalline silicon in HF is reported for the first time. This formation is due to an instability of the dissolution attributed to the high resistivity of the silicon electrode as compared to that of the electrolyte, as it had been observed during anodization of hydrogenated amorphous silicon. This instability is quantitatively studied by a linear stability analysis. The morphology of the macropores is governed by the space-charge region and the effective resistivity of the macroporous material.