Neutron diffraction and ac magnetic susceptibility measurements have been performed to study the magnetic ordering of the Mn spins in polycrystalline Nd0.45Ca0.55MnO3. Three peaks around 260, 90, and 40 K were observed in the temperature dependence of the ac susceptibility. Structural analysis using high-resolution neutron diffraction patterns shows that the peak at 260 K is associated with the charge ordering of the Mn ions. Neutron magnetic diffraction shows that the peak at 90 K originates from the ordering of the Mn spins. Both ferromagnetic and antiferromagnetic couplings between the Mn spins were found. The spin structure in the basal plane consists of tilted ferromagnetic chains (along the  direction) aligned antiferromagnetically, whereas the interlayer coupling is ferromagnetic. A spin reorientation occurs below 15 K, which results in a change in the title angle. The Mn spins order at T-N approximate to 120 K, and the moment saturated at 30 K at [mu(Z)]=2.29(4)mu(B). (C) 1999 American Institute of Physics. [S0021-8979(99)26008-2].