Molluscan adductor muscles are in general of two kinds--a slow-acting smooth muscle, which after contraction can go into a catch state, in which tension is maintained for many hours with a small turnover of ATP, and a quicker-acting muscle, usually to some extent striated, which has not the catch property in marked degree, if at all. The thick filaments of both kinds of adductors contain myosin and paramyosin. Those from catch muscles are noteworthy for their high paramyosin content(up to about 90% by weight), which may be correlated with their great length and diameter. The paramyosin forms a core with a surface layer of myosin. Recently, we have shown that the paramyosin core of catch muscles resembles a single crystal in its molecular arrangement, although the degree of regularity varies from one filament to another. The evidence is from electron microscopy both of individual filaments and of pieces of whole muscle embedded and sectioned. Individual negatively stained filaments change greatly in appearance when rotated round their long axes, in a way not compatible with a helical structure. A three-dimensional reconstruction of the filament from micrographs of a rotated filament confirms this. Transverse sections of smooth adductor muscles show no internal features within the thick filaments when viewed accurately along the filament axis. On tilting striations appear on the filaments that may be correlated with the planes of the Bear-Selby net. They are a general feature of the filaments and have been seen in the smooth adductors of P. maximus, O. edulis, Crassostrea gigas, M. mercenaria, of the anterior byssus retractor of M. edulis and in the red (obliquely striated) adductor of M. mercenaria. The arrangement of the myosin on the paramyosin core is not known.