Mutations in the unc-87 gene of Caenorhabditis elegans cause disorganization of the myofilament lattice in adult bodywall muscle. In order to assess the organization of specific bodywall muscle components in the absence of the unc-87 gene product, we examined the bodywall muscles of mutant animals using phalloidin and monoclonal antibodies to various muscle proteins. These studies indicated that the bodywall muscle of unc-87 embryos is initially almost wild type in its organization, but at later stages, the muscle becomes severely disorganized. To address the possibility that this disorganization is due to deterioration of the muscle as a result of contraction, we introduced into the unc-87 mutant background a mutation that decreases myosin heavy chain activity but does not substantially affect muscle structure. The improved muscle structure and motility of the double mutants are consistent with the hypothesis that at least part of the disorganization phenotype of unc-87 mutants is a consequence of the wild-type levels of force generated during muscle contraction. These results imply that the role of the unc-87 gene product is not in specifying organization but rather in serving as a structural component maintaining lattice integrity during and after contraction.