In actomyosin extracts from smooth muscle obtained at low ionic strength, an assembly of protein into long ribbon-shaped elements is observed to take place. These ribbons which range up to about 100 nm in width and up to many micrometres in length exhibit a strong repeat period of about 5.6 nm. Optical diffraction analysis shows that they possess a long repeat of 39.1 nm <latex>$\pm$</latex> 0.4 nm. Tropomyosin purified from vertebrate smooth muscle can be induced to form the same ribbon-shaped elements. On removal of salt from solution the ribbons dissociate into fine filaments of average diameter about 8 nm which show subfilaments of about 2 to 3 nm diameter. In crude preparations the ribbons occur in solution together with myosin. If such preparations are left to stand for several days, ribbons may be found that show a visible 14 nm period which appears to arise from the presence of a regular arrangement of projections. Smooth muscle myosin alone assembles into cylindrical filaments which exhibit a regular arrangement of projections along their entire length, indicating an absence of polarity. These results indicate, as have those recently obtained from section material, that the myosin-containing component of vertebrate smooth muscle contains a protein that forms the core of the filament, which is responsible for its ribbon-like shape and which probably determines the polarity of the attached myosin molecules. It is proposed that this protein is tropomyosin.