Nematodes construct thick filaments in body-wall muscle cells using three major protein components: two kinds of myosin and paramyosin. We report the isolation and enrichment of thick filaments from wild type and mutant strains of the nematode, Caenorhabditis elegans, and their examination by electron microscopy. We have compared the morphology and distribution of lengths of isolated filaments from the wild type N2, the unc-15 mutant lacking paramyosin E1214 and the unc-54 mutant lacking specific body-wall myosin E190. Our studies indicate that very long filaments can be isolated from both N2 and the E190 lacking 60% of the normal amount of myosin and that the mean length of the isolated filaments and filament fragments in these strains is 3.2 and 3.3 μm, respectively. However, when similar preparations of thick filaments isolated from E1214 were examined, no long filaments were observed and the mean length was 1.0 μm. Most of the E1214 filaments exhibited prominent central bare zones and double tapered ends. The mean length of this normally distributed subpopulation was 1.53 μm. For comparison, we determined the in situ length of wild type N2 thick filaments (by polarized light microscopy of body-wall muscle) to be 9.7 μm. The diameters of the wild type N2 and the myosin mutant E190 thick filaments were 25 nm uniformly along their length, whereas the paramyosin mutant E1214 filaments showed wider diameters of 32 nm along most of their length. We conclude that functional paramyosin is necessary for the determination of thick filament length and diameter in nematode body-wall muscle cells in vivo. The presence of both body-wall myosins and other myofibrillar components appear insufficient for determination of thick filament structure in these muscles. Both body-wall myosins are unnecessary for the formation of long thick filaments of normal diameter: the presence of one form of myosin, paramyosin and other myofibrillar components is sufficient. We discuss the relevance of these findings to thick filament assembly and myofibrillar organization in general.