We have screened available chromosomal deficiencies for genetic loci whose zygotic expression is required for body-wall muscle cells during embryogenesis in Caenorhabditis elegans. In previous studies, it has been shown that no signs of muscle development were detected in nullo-X embryos during embryogenesis. Based on this observation, we are interested in identifying potential loci on X chromosome required for muscle development. Several chromosomal deficiency strains have been analyzed for their terminal phenotypes in order to map such myogenic loci on X chromosome. We have collected homozygotic embryos and observed the terminal phenotypes of the arrested embryos by Nomarski microscopy. These homozygotic embryos will be stained with monoclonal antibodies against muscle specific myosins in order to assess their muscle formation.