Single-walled carbon nanotubes have been produced in a continuous flow gasphase catalytic growth process, in a heated flow of carbon monoxide at pressures of 1--10 atmospheres and temperatures of 800--1200°C. Catalytic cluster nuclei for the nanotubes are generated in situ by thermal decomposition of iron pentacarbonyl. The yield of nanotubes and their diameter distribution can be varied by varying process parameters: nanotubes as small as 0.6 nm in diameter, corresponding to the size of a C60 molecule, have been generated. This process shows great promise for scaling up for bulk production of carbon nanotubes.