Antiferromagnetic resonance has been observed in single crystals of a new cubic antiferromagnet with the Perovskite structure, TlMnF3. The crystals were grown from a melt in a sealed platinumlined molybdenum container under a slightly positive TlF pressure. AFMR at 25 GHz was used to determine TlMnF3 to be an undistorted cubic antiferromagnet at 4.2°K with 〈111〉 easy axes. At 4.2°K the exchange field was determined as HE = 630 kOe, the cubic anisotropy field as HA = -4K 1/3Ms = 6.8 Oe and the linewidth as 80 Oe. The linewidth is mainly due to inhomogeneous strain broadening. From measurements of AFMR field vs axial stress, the Néel temperature has been determined as T N = 76±1°K and the magnetostriction constant as λ100 = -4b1/[3(c11-c12)] = -3.2×10-6 at 4.2°K. The spin-lattice stress coefficient for Mn2+ is C11 = 4.7×10-13 cm/dyn per ion after correcting for the magnetic dipolar contribution. The paramagnetic Curie temperature has been determined from high temperature susceptibility measurements as θ = -138°K. © 1967 The American Institute of Physics.