Magnetization and magnetoresistance of A-type antiferromagnet Pr 0 . 4 5 Sr 0 . 5 5 MnO 3 have been measured by means of pulsed magnetic fields up to 45T. Below T N ~215K, metamagnetic transitions and attendant discontinuous decreases in resistivity were observed. Above the transition fields, the transport anisotropy is reduced compared with that in the antiferromagnetic state. The observed ferromagnetic transition and resistivity change indicate the simultaneous collapse of spin and orbital order by magnetic field.