The ability of globular actin to form filaments and higher-order network structures of the cytoskeleton is essential for cells to maintain their shape and perform essential functions such as force generation, motility, and division. Alterations of actin structures can dramatically change a cell's ability to function. We found that purified and dispersed single wall carbon nanotubes (SWCNTs) can induce actin bundling in cells and in purified model actin systems. SWCNTs do not induce acute cell death, but cell proliferation is greatly reduced in SWCNT-treated cells with an increase in actin-related division defects. Actin, normally present in basal stress fibers in control cells, is located in heterogeneous structures throughout the SWCNT-treated cell. These SWCNT-induced changes in actin structures are seen functionally in multinucleated cells and with reduced force generation. Ex vivo, purified actin filaments cross-linked with alpha-actinin and formed isotropic networks, whereas SWCNTs caused purified actin filaments to assemble into bundles. While purified, isolated SWCNTs do not appear acutely toxic, this subcellular reorganization may cause chronic changes to cellular functions.