The propagation of electron beams injected from a spacecraft into an ambient plasma and the associated potential structures are investigated by one-dimensional Vlasov simulations. For moderate beams, for which the time average spacecraft potential (&PHgr;sa) lies in the range Te≪e&PHgr;sa≲WB, where Te is the electron temperature in energy units and WB is the average beam energy, a double layer forms near the beam head which propagates into the ambient plasma much more slowly than the initial beam velocity. The double layer accelerates the ambient electrons, which carry the return current to the spacecraft. The double layer formation is being reported for the first time. For weak beams, for which ‖ e&PHgr;sa ‖≲Te, the beam propagates with the initial beam velocity, and no double layer formation occurs. On the other hand, for strong beams for which e&PHgr;sa>WB, the bulk of the beam is returned to the spacecraft, and the main feature of the potential structure is a sheath formation with an intense electric field limited to distances <10&lgr;d near the spacecraft surface. These features of the potential structures are compared with those seen in laboratory and space experiments on electron beam injections. ¿ American Geophysical Union 1988