The thick filaments of molluscan muscles have been examined by electron microscopy and X-ray diffraction in order to test whether the structure of the paramyosin core is crystalline and not helicoidal. In accurately cut transverse sections of the white adductor muscle of the oyster the thick filaments are uniformly stained but, when the sections are titled, parallel striations are revealed. Essentially all the filaments can be shown to exhibit striations which can only be explained in terms of a crystalline structure. The crystallinity is, however, not perfect since the striations although often straight are more frequently curved. Two sets of striations of different spacing can be detected in each filament on tilting to either side of the filament axis. Their spacings when compared to the Bear-Selby net a spacing obtained from X-ray diffraction patterns of the embedded muscle indicate that the striations are seen when the filaments are viewed down the 201 and 301 planes of the Bear-Selby net. Striations have been observed in thick filaments from other catch muscles, the white adductor of the clam, the smooth adductor of the scallop and the anterior byssus retractor of the mussel as well as in a non-catch muscle, the red adductor of the clam. In all cases the spacing striations was similar indicating that, contrary to previous reports, when the conditions are the same the a spacing of the Bear-Selby net is approximately constant from one muscle to another and hence that the paramyosin structure in all these muscles is similar.