This PIC code is used for studies of kinetic processes in the terrestrial magnetotail. The code uses an explicit, leapfrog time advance scheme for the fields and particles and is fully relativistic. A Poisson correction for the electric field, based on FFTs and a tridiagonal algorithm, is used to ensure that the continuity equation is preserved on the spatial grid. The parallel implementation is achieved using a 2D nonuniform spatial domain decomposition. The unique feature of the code is the incorporation of a near-Earth plasma sheet configuration containing a dipole field and anasymptotic tail equilibrium with normal field component as well as open boundary conditions along the magnetic field lines that allow for inflow/outflow of magnetic flux and particle populations. Because MAGTAIL shares several components with UPIC (such as the parallel particle managers and the parallel FFT), it has similar scaling properties as UPIC. Furthermore, it can (and will) incorporate new algorithms from UPIC to achieve good scalability for the next generation of supercomputers.