Reversal of the net ferromagnetic moment in an antiferromagnet exhibiting weak ferromagnetism would appear qualitatively to require only small angle rotation of the sublattice moments. However, consideration of the canting mechanism responsible for the weak ferromagnetism shows that it is asymmetric in a way which requires full 180°reversal of the sublattice moments, i.e., M1×M2 retains the same direction in space. This is true for both the Dzialoshinsky-Moriya and single-ion anisotropy canting mechanisms. The critical values of the applied field for reversal of the magnetization and the corresponding initial modes of deviation from equilibrium have been calculated for single-domain particles of antiferromagnetic material having either type of weak ferromagnetic moment. Of the four modes found, two have extremely high critical fields since they involve rotation in opposition of the sublattice moments at the expense of exchange energy. The two reversal modes of interest are (initially) rotations of the Mi about axes parallel and perpendicular to the particle axis, principally at the expense of anisotropy energy. If the initial character of the motion is assumed to persist, the mode Ω⊥ would be a 180°rotation of m≡M 1+M2 and 1≡M1-M2 in the plane of the Mi and the particle axis. The mode Ω∥ would be a 180°rotation of 1 in the plane perpendicular to the particle axis while simultaneously m decreases to zero and then grows to equilibrium value in the opposite direction along the particle axis such that 12+m 2=constant. The critical fields for these two modes are given by the zero-frequency limits of the dynamic resonance equations as pointed out by Brown. In general, the smallest critical fields will be found for the Ω⊥ mode in Dzialoshinsky-Moriya type weak ferromagnets with small, but finite, basal plane anisotropy. The lowest |Hc| calculated is 40 Oe for αFe2O3. © 1964 The American Institute of Physics.