THE interest in carbon nanotubes has been greatly stimulated by theoretical predictions that their electronic properties are strongly modulated by small structural variations(1-8). In particular, the diameter and the helicity of carbon atoms in the nanotube shell are believed to determine whether the nanotube is metallic or a semiconductor, Because of the enormous technical challenge of making measurements on individual nanotubes, however, experimental studies have been limited mainly to bulk measurements(9), which indicate only that a fraction of the nanotubes are metallic or narrow-band semiconductors(10). Recently, measurements of the magneto-conductance of a single multi-shell nanotube in a two-probe configuration showed that the transport is characterized by disorder and localization phenomena(11). To avoid possible ambiguities due to poor sample contacts, four-probe measurements are needed, Here we report four-probe measurements on single nanotubes made by lithographic deposition of tungsten leads across the tubes. We find that each multi-shell nanotube has unique conductivity properties. Both metallic and non-metallic behaviour are observed, as well as abrupt jumps in conductivity as the temperature is varied. The differences between the electrical properties of different nanotubes are far greater than expected. Our results suggest that differences in geometry play a profound part in determining the electronic behaviour.