First magnetocrystalline anisotropy constantsK1∞of 3.6, 6.7, 8.8, and 12.4 wt% silicon in iron alloys have been measured at 77°K and 297°K by determining the torque exerted on {100} single‐crystal disks in uniform magnetic fields. The following results on disordered alloys have been obtained:Composition (wt%Si)Temperature (∘K)K1∞×103(erg cm−3)3.677430±10297360±106.777290±10297205±108.877245±10297165±1012.477120±529778±5The measurements of this investigation are compared with the previously known studies. The agreement, where an overlap occurs, is satisfactory.