Three types of myofilaments in the stomach muscle were studied with an electron microscope, in connection with isometric and isotonic contraction. In dogs, generally, thick and thin filaments were observed in the contracted muscles.In frogs, thick filaments were found only in the contracted muscles of which myoplasm invariably contained calcium (oxalate or pyroantimonate) deposits.In frog stomach, contracted muscles often contained lattice-like arrangements of thin filaments, suggesting molecular changes. Dense bodies were often contracted and associated with contraction of thin filaments.It was postulated that the dense bodies-thin filaments system is functionally equivalent to the sarcomere of the striated muscles. Both contraction-relaxation cycles and formation of thick filaments are probably regulated by the intracellular concentration of free calcium.