The room- temperature structures of the BiMnO3 ceramic sample synthesized under high pressure have been investigated using transmission- electron microscopy ( TEM) and theoretical simulations. The electron diffraction ( ED) data revealed that the stoichiometric BiMnO3 crystallizes in the centrosymmetric C-2/ c structure, and not in the noncentrosymmetric C2 structure. This was confirmed by further calculations of high- resolution transmission- electron microscopy ( HRTEM) images. A second polymorph, characterized as a body- centered pseudocubic superstructure with the lattice constants of 4a(p) x 4b(p) x 4c(p), was also found in the present sample. Composition analyses and image simulations revealed that this superlattice was formed as the result of an ordered oxygen deficiency. In terms of crystallography, we suggest that the weak ferroelectric polarizations measured on the BiMnO3 samples originate from the superlattice. This work is very important for further studies of BiMnO3 and structurally related compounds.