Carbon tubes or fibres with diameters ranging from 0.05 to 1 mu m have been produced in a hot isostatic pressure apparatus, using gas pressures up to 100 MPa. Nitrogen, argon or mixtures thereof have been used. Other carbon structures were also present in small amounts. Transmission electron microscopy revealed tubes of cylindrical and conical shape. The semi-apex angles for the conical tubes were measured to be close to 5, 10 and 20 degrees. A possible growth mechanism for the tubes of conical shape is proposed.