  The problem of Bloch electrons in two dimensions subject to magnetic and
intense electric fields is investigated. Magnetic translations, electric
evolution and energy translation operators are used to specify the solutions of
the Schr\"odinger equation. For rational values of the magnetic flux quanta per
unit cell and commensurate orientations of the electric field relative to the
original lattice, an extended superlattice can be defined and a complete set of
mutually commuting space-time symmetry operators is obtained. Dynamics of the
system is governed by a finite difference equation that exactly includes the
effects of: an arbitrary periodic potential, an electric field orientated in a
commensurable direction of the lattice, and coupling between Landau levels. A
weak periodic potential broadens each Landau level in a series of minibands,
separated by the corresponding minigaps. The addition of the electric field
induces a series of avoided and exact crossing of the quasienergies, for
sufficiently strong electric field the spectrum evolves into equally spaced
discreet levels, in this "magnetic Stark ladder" the energy separation is an
integer multiple of $ h E / a B $, with $a$ the lattice parameter.
