AbstractA search for new mutants with altered body‐wall muscle cell structure has been undertaken in the nematode C elegans. One‐hundred seventeen mutants were isolated after mutagenesis with ethyl methanesulfonate or ultraviolet light, enrichment by a motility‐requiring test, and screening by polarized light microscopy; 102 of these mutants were in ten previously established genes, whereas 15 mutants permitted the identification of seven new complementation groups in C elegans. Two of the new genes map on linkage group I (unc‐94andunc‐95) and four genes are sex linked (unc‐96, unc‐97, unc‐98, andunc‐99). One complementation group (unc‐100) could not be mapped because of the special characteristics of its cohort mutants. Representative mutants of the mapped genes were examined by polarized light and electron microscopy. All of the mutants exhibit disruptions of the normal A and I band organization of thick and thin filaments. Several of the mutants produce collections of thin filament‐like structures. In one of these cases, HE177 demonstrated collections of somewhat wider, intermediate‐sized filaments as well, and the HE195 mutant produces paracrystalline aggregates of thin filaments amidst looser arrangements of similar structures. The mutants in newly identified genes, as well as the new mutants in previously established genetic loci, have promise as tools in the study of myofibrillar assembly and function. Among the 22 complementation groups associated with body‐wall structure in C elegans, it is likely that some genes code for regulatory and morphogenetic functions in addition to the well‐studied structural, contractile, and calcium‐