Single crystals of bismuth of 170 mm diameter and 35 kg weight were grown by directional solidification. The nucleation and orientation of the seed crystal was performed in situ on a graphite sensor. Due to the strong anisotropy in the heat conductivity the better conducting c-plane is aligned with the heat flow nearly parallel to the crucible axis, thus permitting to cut [c]- oriented neutron filter of maximum size. The single crystals are used as windows for subthermic neutrons and filter against 7-rays. The transmission spectra for crystals with weak - and strong - mosaic spread were measured.