One aspect of Argonne research in parallel computing involves the speed and other properties of parallel SDI algorithms. Various algorithms under study have exhibited speedups resulting from parallelization on shared-memory machines. A weapon-target accessibility algorithm called ACCESS exhibited a high degree of inherent parallelism and has been studied on a wide variety of sequential and parallel multiple instruction multiple data (MIMD) machines. To study ACCESS on a massively parallel single instruction multiple data (SIMD) machine architecture, ANL researchers developed a version of ACCESS on a Thinking Machines Corporation 16K processor Connection Machine-2 (CM-2) located at the ACRF. ANL researchers wrote the Connection Machine version of ACCESS in C(*), a version of C by Thinking Machines Corporation with extensions to accommodate SIMD parallelism. Because of the large number of available physical processors and the ability to create virtual processors on the CM-2, the Connection Machine version of ACCESS was able to process an array of 128 x 1024 tasks in parallel. For the data tested, the CM-2 implementation of ACCESS was faster than both the parallel version run on the Alliant FX/8, the Encore Multimax, and the Sequent Balance and the sequential version run on the ANL Cray X-MP/14. For the benchmark ACCESS problem, the CM-2 at ANL with 16K processors achieved a sustained performance of 400 Mflops. On other larger CM-2 machines, the same problem achieved even higher performance: nearly 1600 Mflops on the Los Alamos National Laboratory 64K processor CM-2. %boratory 64K processor CM-2. The investigation has demonstrated that achieving optimal performance requires structuring the code carefully to keep all available processors busy and to reduce disruptive communication on the front-end processor