The defect structure at the growth front of 4H-SiC single crystal boules grown by the physical vapor transport (PVT) growth method has been investigated using X-ray topography. Plan-view observations of the growth front of the boules revealed that basal plane dislocations (BPDs) were often nucleated at the outcrops of threading screw dislocations during growth. The observations also revealed that BPDs introduced in the facet and nearfacet regions were hardly multiplied during the PVT growth process, and thus the crystal portions just beneath these regions contained very low densities of BPDs. The cross-sectional observations of the grown boules confirmed this result and further revealed that a number of BPDs were introduced in the crystal portions well distant from the growth front. They are thought to be introduced via dislocation multiplication due to large thermoelastic stresses imposed on the shoulder parts of the grown boules during PVT growth.