The symmetric multiprocessor (SMP) appears as a unit in systems from desktop computers to massively parallel systems. The focus of this paper is the performance of a particular algorithm, two-dimensional semicoarsening multigrid, on a cluster of SMPs. Complexity models for hybrid parallelization of a portion of this algorithm are derived. We examine system parameters, such as the capability for simultaneous message-passing in a symmetric multiprocessor, that can affect the performance of hybrid code. Complexity estimates are tested for a variety of decomposition strategies, line solve methods, and problem sizes. Results from the Intel Teraflops supercomputer and from a Beowulf cluster of dual-processor Xeons are presented. An expanded abstract in PostScript form has been sent to MGNet.