Changes in the crystal and magnetic structures of BiFeO3 produced by substitution of some Fe ions by Mn ions are considered. For BiMnxFe1-xO3 with x = 0.1 and 0.2, the refinements of the crystal and magnetic structure parameters from time-of-flight (TOF) neutron diffraction data collected at 293 K are presented. The magnetic ordering temperatures are 670(5) K, 610(5) K and 560(5) K for x = 0.0, 0.1, and 0.2, respectively. The mixed compounds exhibit unusual magnetization curves indicating a considerable degree of frustration involved in the magnetic ordering process.