The role of random uniaxial anisotropy on critical behavior has been studied in the antiferromagnetic system FexMn1−xF2using linear magnetic birefringence. In the pure FeF2and MnF2crystals the deduced magnetic specific heat near TNshows no rounding for reduced temperature t as small as 10−4. Except for the effects of concentration gradients, no rounding is observed in the mixed crystals. In fact, for the Fe.75Mn.25F2sample, where such gradient effects are minimal, the transition is as sharp as in pure FeF2. Since MnF2and FeF2differ primarily in the size of the single ion anisotropy, it is clear that random anisotropy, by itself, does not broaden the transition. If anisotropy were not considered, MnF2and FeF2would be expected to have nearly identical TN. Although MnF2is itself asymptotically Ising‐like as T→TNbecause of its small but finite anisotropy, the larger TNin the more anisotropic FeF2and the drastic increase in TNwith x near x = 0 is attributable to the effects of crossover from Heisenberg to Ising critical behavior.