EPR (X-band) study of Gd³⁺-doped (0.1 mol%) La_0.9Nd_0.1F₃ single crystal at liquid helium, liquid nitrogen and room temperatures has been carried out in order to determine the crystal field effects in the La_0.9Nd_0.1F₃ host, and to estimate the exchange interactions between Gd³⁺ and Nd³⁺ ions. Eleven spin-Hamiltonian parameters have been evaluated, by fitting the data from the angular variation of resonance lines in the ZX plane, to the spin-Hamiltonian for the rhombic C_2v symmetry of the crystal field. The average Gd³⁺-Nd³⁺ exchange interaction J (= -2.7 GHz) has been estimated from the molecular-field model using the Gd³⁺ g-value shift between paramagnetic (Gd³⁺-doped La_0.9Nd_0.1F₃) and diamagnetic (Gd³⁺-doped LaF₃) crystals at 4.2 K.