Abstract The formation of dislocation pile-ups and related small-angle subgrain boundaries in block-cast multicrystalline silicon for photovoltaic applications has been studied by means of white-beam X-ray topography (WB-XRT). For this purpose, samples sliced perpendicular and parallel to the growth direction have been investigated in reflection and transmission geometry, respectively. During the growth process of the silicon ingot, the dislocation density increases. WB-XRT measurements revealed the formation of small-angle subgrain boundaries. The subgrains have a slightly changed orientation related to a rotation of ∼0.07–0.80° around an axis parallel to the growth direction. This tilt results from the high number of dislocations forming dislocation pile-ups and walls. The spacings between dislocations in such subgrain boundaries were found to be between 297 and 28nm. A qualitative model for the formation of dislocation pile-ups is proposed.