We investigated magnetic domain structures of a multiferroic manganite, BiMnO3, by Lorentz transmission electron microscopy. Ferromagnetic domains were observed below similar to 105 K, close to the ferromagnetic Curie temperature, T-C. The spontaneous magnetization aligns distinctly along the [010] direction, suggesting that the magnetic easy direction is along the b axis. Inflection and merging of the domain walls was observed at twin boundaries. This indicates pinning of the magnetic domain walls at crystallographic twin boundaries. Furthermore, we observed narrow magnetic domain walls, suggesting strong magnetocrystalline anisotropy. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4742747]