The crystal and magnetic structures of polycrystalline BiCoO3 have been determined by the Rietveld method from neutron diffraction data measured at temperatures from 5 to 520 K. BiCoO3 (space group P4mm; Z = 1; a = 3.72937(7) (A) over circle and c = 4.72382(15) (A) over circle at room temperature; tetragonality c/a = 1.267) is isotypic with BaTiO3 and PbTiO3 in the whole temperature range. BiCoO3 is an insulator with a Neel temperature of 470 K. A possible model for antiferromagnetic order is proposed with a propagation vector of k = ((1)/(2), (1)/(2), 0). In this model, magnetic moments of Co3+ ions are parallel to the c direction and align antiferromagnetically in the ab plane. The antiferromagnetic ab layers stack ferromagnetically along the c axis, forming a C-type antiferromagnetic structure. Refined magnetic moments at 5 and 300 K are 3.24(2)mu(B) and 2.93(2)mu(B), respectively. The structure refinements revealed no deviation from stoichiometry in BiCoO3. BiCoO3 decomposed in air above 720 K to give Co3O4 and sillenite-like Bi25CoO39.