A parallel algorithm for angular domain decomposition (or parallelization) of an r-depen-dent spherical Sn transport theory method is derived. The parallel formulation is incorporated into TWOTRAN-II using the IBM Parallel FORTRAN compiler and implemented on an IBM 3090/400 (with four processors). The behavior of the parallel algorithm for different physical problems is studied, and it is concluded that the parallel algorithm behaves differently in the presence of a fission source as opposed to the absence of a fission source; this is attributed to the relative contributions of the source and the angular redistribution terms in the Sn algorithm. Further, the parallel performance of the algorithm is measured for various problem sizes and different combinations of angular subdomains or processors. Poor parallel efficiencies between ∼ 35 and 50% are achieved in situations where the relative difference of parallel to serial iterations is ∼ 50%. High parallel efficiencies between ∼ 60% and 90% are obtained in situations where the relative difference of parallel to serial iterations is <35%.