Antiferromagnetic resonance in single crystals of FeF2 was observed between 1.5° and 66°K (TN=78.4°K) in the far-infrared region. The resonance frequency at T~0 was found to be ν~(0)=52.7+/-0.2 cm-1. Using the antiferromagnetic resonance relation derived by Kittel, Nagamiya, Keffer, and others, and using the experimental value for the static susceptibility, the uniaxial anisotropy constant at absolute zero, K(0), was inferred to be 1.1×108 ergs/cm3 (40 cm-1/atom). Measurements of the splitting of the line caused by an external magnetic field gave g∥=2.25+/-0.05. The anisotropy energy in FeF2, being primarily due to the crystalline field-spin-orbit interaction, may be described by a term iDSzi2 in the Hamiltonian. By including this interaction in the molecular-field treatment, we have determined D=-9+/-2 cm-1, and have calculated the temperature dependences of the sublattice magnetization, the anisotropy constant, the resonance frequency, and the line width. The last two were compared with the experimental results and found to be in reasonable agreement. The linewidths were found to follow a T4 law above 15°K. The search over a frequency region of 13-70 cm-1 for the absorption lines expected in the paramagnetic region was unsuccessful, possibly indicating a relaxation time of less than 10-12 sec.