The authors have measured the electrical properties of individual bundles, or "ropes," of single-walled carbon nanotubes. They observed a suppression of low bias conductance below 10 K and dramatic peaks in conductance as a function of gate voltage, which modulates the number of electrons in the rope. These results are interpreted as evidence for single electron charging and resonant tunneling through the quantized energy levels of the nanotubes. The device geometry consists of a single nanotube rope with lithographically defined leads, and the measurements were performed on a 12 nm diameter rope containing approximately 60 single-walled nanotubes. The data show a strong suppression of conductance near zero voltage at low temperatures, and the conductance consists of a series of sharp peaks separated by regions of very low conductance. The peak spacing is determined by the energy to add an additional electron to the dot, and the peak amplitudes vary widely. The authors interpret these results using a Coulomb blockade model, inferring typical addition energies and level spacings from the data. They also discuss the implications of their findings and suggest further experiments to explore the system's properties.The authors have measured the electrical properties of individual bundles, or "ropes," of single-walled carbon nanotubes. They observed a suppression of low bias conductance below 10 K and dramatic peaks in conductance as a function of gate voltage, which modulates the number of electrons in the rope. These results are interpreted as evidence for single electron charging and resonant tunneling through the quantized energy levels of the nanotubes. The device geometry consists of a single nanotube rope with lithographically defined leads, and the measurements were performed on a 12 nm diameter rope containing approximately 60 single-walled nanotubes. The data show a strong suppression of conductance near zero voltage at low temperatures, and the conductance consists of a series of sharp peaks separated by regions of very low conductance. The peak spacing is determined by the energy to add an additional electron to the dot, and the peak amplitudes vary widely. The authors interpret these results using a Coulomb blockade model, inferring typical addition energies and level spacings from the data. They also discuss the implications of their findings and suggest further experiments to explore the system's properties.