  Monopole-mediated baryon number violation, the Callan-Rubakov effect, is
reexamined using boundary conformal field theory techniques. It is shown that
the low-energy behaviour is described simply by free fermions with a
conformally invariant boundary condition at the dyon location. When the number
of fermion flavours is greater than two, this boundary condition is of a
non-trivial type which has not been elucidated previously.
