  We describe a parallel, cosmological N-body code based on a hybrid scheme
using the particle-mesh (PM) and Barnes-Hut (BH) oct-tree algorithm. We call
the algorithm GOTPM for Grid-of-Oct-Trees-Particle-Mesh. The code is
parallelized using the Message Passing Interface (MPI) library and is optimized
to run on Beowulf clusters as well as symmetric multi-processors. The
gravitational potential is determined on a mesh using a standard PM method with
particle forces determined through interpolation. The softened PM force is
corrected for short range interactions using a grid of localized BH trees
throughout the entire simulation volume in a completely analogous way to P$^3$M
methods. This method makes no assumptions about the local density for short
range force corrections and so is consistent with the results of the P$^3$M
method in the limit that the treecode opening angle parameter, $\theta \to 0$.
  (abridged)
