A localization model comprising spin disorder and nonmagnetic randomness is presented to account for novel magnetotransport properties in Mn oxides R12xAxMnO3. Localization length of electrons as a function of magnetization is determined by means of the transfer matrix method. Including the Coulomb interaction between electrons, the variable-range hopping resistivity is calculated as a function of temperature and magnetic field. The resulting sharp resistivity peak near the Curie temperature and, in particular, the magnitude of the colossal negative magnetoresistance are in good agreement with experimental measurements.