This is gather-scatter software for message passing implementation of matrix-vector products for domain decomposition based iterative solvers. It has a particularly simple interface in which the user provides a list of global indices in a preprocessing phase, followed by the vectors to be summed (the gather-scatter operation) in subsequent execution steps. The package can be called from C or Fortran and supports either the Intel NX or MPI message passing protocols. C and Fortran example drivers and makefiles are provided. Code developed by Henry Tufo and Paul Fischer with support from the NSF under Grant ASC-9405403 and the AFOSR under Grant F49620-95-1-0074.