The unc-54 gene of Caenorhabditis elegans encodes an abundant myosin heavy chain protein expressed in body-wall muscle cells. We have designed genetic techniques that select directly for unc-54 mutants. This selection is based upon properties of the unc-54 dominant allele e1152. Mutations that eliminate dominance of e1152 are null alleles of unc-54. Deletions have been identified by their genetic properties. We have defined mutationally a number of essential genes near unc-54, and we have described the genetic fine structure of this region of linkage group I. As much as 27% of the unc-54 mutations induced by the bifunctional alkylating agent 1,2,7,8-diepoxyoctane are multisite deletions. Extrachromosomal free duplications that include unc-54 are also described.