  It is shown that the total set of equations, which determines the dynamics of
the domain bounds (DB) in a weak ferromagnet, has the same type of specific
solution as the well-known Walker's solution for ferromagnets. We calculated
the functional dependence of the velocity of the DB on the magnetic field,
which is described by the obtained solution. This function has a maximum at a
finite field and a section of the negative differential mobility of the DB.
According to the calculation, the maximum velocity $ c \approx 2 \times 10^6$
cm/sec in YFeO$_3$ is reached at $H_m \approx 4 \times 10^3$ Oe.
