Fully developed flows of a viscous incompressible isotropically conducting fluid are investigated in a rectangular channel in the presence of a transverse magnetic field. An exact solution of the problem is obtained in general form and for the limiting case corresponding to a flow in a plane slit. It is shown that at high Hartmann numbers a region of elevated velocities can form near the axis of the channel. In conclusion, some other flows in inhomogeneous fields with tapered geometry are considered.