The present work is devoted to the theoretical investigation of magnetoresonance properties of lanthanium manganite, which is a parent compound of CMR materials. Single-ion anisotropy constants and g-tensor components are found. Dependencies of exchange parameters, single-ion anisotropy, and g-tensor components upon the orbital structure are shown. Rotational distortions are also taken into account. The exchange parameters of other rare-earth orthorhombic manganites RMnO3 (R = Pr, Nd, Tb, Dy, Ho, Y, Er) are estimated as well.