  Many years and great effort have been spent constructing the microscopic
model for the room temperature multiferroic BiFeO3 However, earlier models
implicitly assumed that the cycloidal wavevector q was confined to one of the
three-fold symmetric axis in the hexagonal plane normal to the electric
polarization. Because recent measurements indicate that q can be rotated by a
magnetic field, it is essential to properly treat the anisotropy that confines
q at low fields. We show that the anisotropy energy $-K_3 S^6 \sin^6 \theta
\cos 6 \phi $ confines the wavevectors q to the three-fold axis $\phi =0$ and
$+-2 \pi/3$ within the hexagonal plane with $\theta = \pi /2$.
