  We consider antiferromagnets breaking both time-reversal (Theta) and a
primitive lattice translational symmetry (T) of a crystal but preserving the
combination S = Theta T. The S symmetry leads to a Z_2 topological
classification of insulators, separating the ordinary insulator phase from the
"antiferromagnetic topological insulator" (AFTI) phase. This state is similar
to the "strong" topological insulator with time-reversal symmetry, and shares
with it such properties as a quantized magnetoelectric effect. However, for
certain surfaces the surface states are intrinsically gapped with a
half-quantum Hall effect (sigma_{xy} = e^2 / 2h), which may aid experimental
confirmation of theta = pi quantized magnetoelectric coupling. Step edges on
such a surface support gapless, chiral quantum wires. In closing we discuss
GdBiPt as a possible example of this topological class.
