The body wall muscle cells of Caenorhabditis elegans are a developmental system in which questions of gene expression in determination of muscle cell fates and in differentiation to produce functioning myofibrillar contracile units are examined by genetic, molecular, and cellular techniques. These approaches have been singularly useful in understanding the requirements of nonmyosin proteins and activities in the assembly of myosin into thick myofilaments and of membrane and extracellular proteins in the organization of myofilaments into ordered