Smooth muscle cells may be isolated from the taenia coli muscle of the guinea pig which, when made permeable by treatment with Triton X-100 (0-05%) show a sensitivity to Ca for contraction with MgATP. The rate of contraction, about 10 micron s-1, corresponds closely to the maximum velocity of shortening of the intact muscle. Electron microscopy of such partially demembranated muscle cells shows that myosin filaments of about 16-nm diameter are present in both the rigor and the relaxes states. In addition, the actin and myosin filaments are commonly seen to be associated in groups corresponding approximately in size to the fibrils recognizable in cells in rigor in the light microscope. The dense bodies and the 10-nm filaments are found located between the actin-myosin filament groups. The thick myosin filaments may be isolated by fragmentation of the cells under relaxing conditions. These native filaments range up to about 8 micron in length and show the same structural organization as filaments aseembled from purified smooth muscle myosin: there is no central bare zone and bare edges, about 0-2 micrin long, occur at the filament ends. The lack of bipolarity of the native smooth muscle muosin filaments and the absence, in the contractile apparatus, of actin-associated structures equivalent to Z-lines suggests that the amount of shearing that can occur between the actin and myosin filaments is considerably greater than in skeletal muscle.