  We examine solitary waves in classical Heisenberg chains with an uniaxial
anisotropy and a parallel magnetic field in a continuum approach. The boundary
conditions commonly used are generalized to nonlinear spin wave states, which
themselves turn out to be stable only for an anisotropy of the easy-plane type.
In this case we obtain two different branches of one-soliton solutions which
can be mapped onto each other by a formal time inversion. Moreover, they show
some remarkable similarity to dark solitons of the Nonlinear Schr\"odinger
equation. Numerical simulations for the discrete Heisenberg chain show that
these solitary waves are highly, but not absolutely stable under interaction
with linear excitations and as well under scattering with each other. The
possible significance of these solitary waves in a phenomenological theory of
one-dimensional magnets is briefly addressed.
