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BlockSolve is a software library for solving large, sparse systems of linear equations on massively parallel computers. The matrices must be symmetric, but may have an arbitrary sparsity structure. BlockSolve is a portable package that is compatible with several dierent message-passing pardigms. This report gives detailed instructions on the use of BlockSolve in applications programs. This work was supported by the Oce of Scientic Computing, U.S. Department of Energy, under Contract W-31-109-Eng-38.