  We derive fully explicit equations of motion, and the associated set of
constraints, describing the propagation in a flat space-time of a charged
spin-3/2 massive state in a constant electromagnetic background. For this
purpose, we provide the Lagrangian for the physical fermionic fields in the
first massive level of the open superstring. We first write a compact
Lagrangian, allowing a simple derivation of the equations of motion and
constraints. Then another one is given that yields directly a decoupled system
of equations, though the fields of different spins look coupled at the level of
the Lagrangian.
