Single crystals of cobalt‐doped thallium manganese fluoride (TlMn1−xCoxF3) have been studied by antiferromagnetic resonance for cobalt concentrations of 0≤x≤0.001. In this range the antiferromagnet remains cubic at 4.2°K, with its crystalline anisotropy varying linearly with cobalt concentration fromK1/M= −5.1 Oe for pure TlMnF3to +8.2 Oe, passing through zero forx=0.0004. Other magnetic properties, such as Ne´el temperature and exchange field, are unaffected by the small cobalt concentrations. Multiple non‐equivalent equilibrium spin‐orientation states, which have been observed in spin‐flopped resonance in the ⟨100⟩ and ⟨110⟩ crystal planes whenK1/Mis greater than zero, are described.