The thick filaments of the nematode Caenorhabditis elegans contain two myosin heavy chain isoforms A and B and paramyosin, the products of the myo-3, unc-54, and unc-15 genes, respectively. Dissociation of paramyosin from native thick filaments at pH 6.36 shows a biphasic function with respect to NaCl concentration. Electron microscopy of the remaining structures shows 15-nm core structures that label with monoclonal anti-paramyosin antibody at 72.5-nm intervals. Purified core structures also show 72.5 nm repeats by negative staining. Structural analysis of native thick filaments and dissociated structures suggests that the more dissociable paramyosin is removed radially as well as processively from the filament ends. Minor proteins with masses of 20, 28, and 30 kD cosediment stoichiometrically with paramyosin in purified core structures.