Treatment of spread, cultured cells with Triton X-100 followed by negative staining reveals the organization of the unextracted intracellular filamentous elements: actin, microtubules and the 100 angstrom filaments. The present report describes the organization of the actin-like filaments in human skin fibroblasts and mouse 3 T 3 cells. As shown in earlier studies, the cytoplasmic stress fibres were seen to be composed of bundles of colinear actin-like filaments. In addition to these large stress fibres much smaller bundles of thin filaments as well as randomly oriented thin filaments were also observed. A thick bundle of thin filaments, 0.2 microm to 0.5 microm in diameter, was found to delimit the concave cell edges most prominent in well-spread stationary cells. The leading edge and ruffled border of human skin fibroblasts appeared as a broad web, of meshwork of diagonally oriented thin filaments interconnecting radiating, linear bundles of thin filaments about 0.1 microm in diameter. These bundles corresponding to the microspikes described earlier ranged from about 1.5 microm in length and were separated by 1 microm to 3 microm laterally. The leading edge of 3 T 3 cells showed a similar organization but with fewer radiating thin filament bundles. Both the filaments in the bundles and in the meshwork formed arrowhead complexes with smooth muscle myosin subfragment - 1 which were unipolar and directed towards the main body of the cell. The findings are discussed in relation to the mechanisms of non-muscle cell motility.