The paramagnetic resonance of an impurity in single crystals of BaTiO3 has been studied. The impurity ion has been identified as Fe3+ and is believed to be located at the titanium position in the unit cell. The zero field splittings for the Fe3+ ion in the tetragonal ferroelectric phase have been measured at room temperature and as a function of temperature through the Curie point. The values at room temperature are 11.7 and 4.9 kmc s, respectively. The corresponding zero field splitting in the cubic phase is about 0.90 kmc. The effects of temperature variation on the resonant magnetic fields are discussed with special reference to the observation of a zero magnetic field transition at 9.5 kmc s and at a temperature of 75°C. © 1959.