EPR studies on diamagnetic LaNbO4 and paramagnetic PrNbO4 crystals doped with the Kramers ions Nd3+ and Ce3+ were carried out at X-band (9.61 GHz) in the 4-30 K temperature range. The principal values of Nd3+ and Ce3+ g(2) and A(2) tensors, and the orientations of their principal axes, were determined accurately from angular variations of EPR line positions recorded for rotation of the external magnetic field in three mutually perpendicular planes in the two crystals at 8 K. The larger EPR line widths of these ions in the paramagnetic PrNbO4 host crystal as compared to those in diamagnetic LaNbO4 are ascribed to their interaction with the host Pr3+ ions and presence of defects.