By taking advantage of a lethal phenotype characteristic of Caenorhabditis elegans embryos that fail to move, we have identified 13 genes required for muscle assembly and function and discovered a new lethal class of alleles for three previously known muscle-affecting genes. By staining mutant embryos for myosin and actin we have recognized five distinct classes of genes: mutations in four genes disrupt the assembly of thick and thin filaments into the myofilament lattice as well as the polarized location of these components to the sarcolemma. Mutations in another three genes also disrupt thick and thin filament assembly, but allow proper polarization of lattice components based on the myosin heavy chain isoform that we analyzed. Another two classes of genes are defined by mutations with principal effects on thick or thin filament assembly into the lattice, but not both. The final class includes three genes in which mutations cause relatively minor defects in lattice assembly. Failure of certain mutants to stain with antibodies to specific muscle cell antigens suggest that two genes associated with severe disruptions of myofilament lattice assembly may code for components of the basement membrane and the sarcolemma that are concentrated where dense bodies (Z-line analogs) and M-lines attach to the cell membrane. Similar evidence suggests that one of the genes associated with mild effects on lattice assembly may code for tropomyosin. Many of the newly identified genes are likely to play critical roles in muscle development and function.