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After irradiation, iron (l), nickel (2), o r cobalt (3) presents an important magnetic after-effect zone which anneals out at the end of the stage I of the resistivity recovery. It was attributed, either to the one-dimensional migration of the crowdion (4) which reduces the permeability by a Dietze mechanism (5), o r to the three-dimensional migration and reorientation of a split interstitial. In this case, the magnetic after effect results from a NBel mechanism (6).Magnetic torque measurements made under saturating magnetic field can answer this question. This paper gives the results for iron.Experiments The samples are discs of polycristalline high purity iron ( @ 7 mm; e = 0.2 mm). They were irradiated, at 20 K, by 3 MeV electrons (dose 10l8 electrons, cm ) and transferred, at 77 K, into a torque measurement apparatus, which operates under saturating magnetic field (3400 Oe). Magnetic anisotropy is studied by measuring the torque exerted on the disc by a magnetic field applied in the plane of the disc successively along 18 different directions making angles 0 with an arbitrary diameter used a s origin. Measurements are made at a reference temperature T = 92 K , at which no evolution of the measured torques is observed.In the first experiment, the sample undergoes an isothermal treatment at T = 103 K , the magnetic field being at 0 . At given times, the thermal treatment is stopped, the temperature i s lowered down to 92 K, and measurements a r e made a s described above. The results show that, during the thermal treatment, a uniaxial 0 magnetic anisotropy develops slowly. This anisotropy can be written