The development of single- walled carbon nanotubes for various biomedical applications is an area of great promise. However, the contradictory data on the toxic effects of single- walled carbon nanotubes(1-10) highlight the need for alternative ways to study their uptake and cytotoxic effects in cells. Single- walled carbon nanotubes have been shown to be acutely toxic(1-3) in a number of types of cells, but the direct observation of cellular uptake of single- walled carbon nanotubes has not been demonstrated previously due to difficulties in discriminating carbon- based nanotubes from carbon- rich cell structures. Here we use transmission electron microscopy and confocal microscopy to image the translocation of single- walled carbon nanotubes into cells in both stained and unstained human cells. The nanotubes were seen to enter the cytoplasm and localize within the cell nucleus, causing cell mortality in a dose dependent manner.