Myosin filaments in smooth muscle cells of the guinea pig taenia coli were studied by freeze-substitution electron microscopy combined with rapid freezing using liquid helium. The thick myosin filaments were reproducibly observed both in relaxed and contracted states. In relaxed states, the thick filaments are more or less lined up among the packed bundles of thin filaments. The basic arrangement and relative ratio of thick and thin filaments were the same between the relaxed and contracted cells, except that many of thick filaments were often observed to be very close to thin filaments in contracted states. The results indicate that myosin in smooth muscle cells exists as thick filaments at any stages during contraction-relaxation cycle.