A three-dimensional electrostatic particle simulation model is developed for studying the injection of an electron beam from an isolated source (the ''spacecraft'') in the ionosphere. The dimensionless beam velocity in the simulations is determined by matching the beam energy and injection current to representative experimental values. It is found that the beam stagnation time ts is typically nearly an order of magnitude longer than in previous two-dimensional simulations. As a result, under normal conditions ts is more than an order of magnitude larger than the plasma response time trp, and the beam is able to escape from the near environment of the spacecraft. The limit on the amount of current that can be injected is provided by the condition ts>2trp, and this limit can substantially exceed the thermal current to the spacecraft. After escape, the beam is subject to strong space-charge oscillations produced by the beam-plasma interaction. For injection parallel to the magnetic field this interaction produces a series of trapping vortices in phase space in which the peak potentials reach 10--15% of the beam voltage. For injection at 45¿ to the magnetic field, the beam initially forms a hollow cylinder whose radius is the beam gyroperiods, however, the space-charge oscillations lead to a loss of both the parallel and transverse coherence of the beam. Inside the beam cylinder the wave spectrum is dominated by a peak just below the upper hybrid frequency; outside the cylinder the most intense modes are at lower frequencies near the ion plasma frequency. ¿ American Geophysical Union 1991