Electron microscope examination of myosin-extracted psoas muscle of the rabbit reveals a number of thin filaments, about 25 Ä in diameter, bridging the gap between the ends of the actins and the remaining myosin of the M band. Extraction of actin from the region of the Z disc shows similar thin filaments near the Z disc. Myosin-extraction of several invertebrate muscles reveals a remaining network of thin filaments in the region of the A band. These thin filaments are probably elastic, and could contribute to the functioning of these muscles.