We have isolated three novel alleles of the unc-45 locus in C. elegans, that are recessive lethals. Two of these alleles, when homozygous, result in a nearly total loss of muscle contraction with a concomitant arrest of development and a displacement of muscle cells. The third allele is similar, but showed maternal rescue by a wild-type allele. All previously identified unc-45 alleles were temperature sensitive and, although they produced paralysis of adult animals, all were homozygous viable. Prior genetic studies with these temperature sensitive alleles had suggested that at least one function of the unc-45 gene product was to interact with the major myosin heavy chain isoform, MHC B, of body wall muscles. Our observations of the lethal alleles suggest that the unc-45 product normally interacts with additional muscle components in both the body wall and pharyngeal muscles. In particular, we suggest that the unc-45 product might interact with all four myosin heavy chains: MHC B; MHC A; and the pharyngeal isoforms, MHC C and MHC D. Maternal rescue of the lethality of the third allele shows that the unc-45 gene product is present in the oocytes, although it may not be necessary until late in development when myofilaments begin to assemble.