Carbon fibres have been prepared by pyrolysing a mixture of benzene and hydrogen at about 1100°C. They have been studied by high resolution electron microscopy. These fibres have various external shapes and contain a hollow tube with a diameter ranging from 20 to more than 500 Å along the fibre axis. They consist of turbostratic stacks of carbon layers, parallel to the fibre axis, and arranged in concentric sheets like the "annual ring structure of a tree". These fibres have two textures resulting from different growth processes; core regions, made of long, straight and parallel carbon layers, are primarily formed by catalytic effect; the external regions correspond to a pyrolytic deposit occuring during the secondary thickening growth process. Very small cementite crystals, typically about 100 Å in a diameter, have been identified by dark-field techniques at the tip of the central tube of each fibre. A model of fibre growth related to a surface diffusion of carbon species on the catalyst particle has been established.