Carbon nanotubes(1,2) might be usefully employed in nanometre-scale engineering and electronics. Electrical conductivity measurements on the bulk material(3,4) on individual multi-walled(5,6) and single-walled(7) nanotubes and on bundles of single-walled nanotubes(8,9) have revealed that they may behave as metallic, insulating or semiconducting nanowires, depending on the method of production-which controls the degree of graphitization, the helicity and the diameter. Measurements of Young’s modulus show(10) that single nanotubes are stiffer than commercial carbon fibres. Methods commonly used to generate nanotubes-carbon-arc discharge techniques(1,2,4), catalytic pyrolysis of hydrocarbons(11,12) and condensed-phase electrolysis(13,14)-generally suffer from the drawbacks that polyhedral particles are also formed and that the dimensions of the nanotubes are highly variable. Here we describe a method for generating aligned carbon nanotubes by pyrolysis of 2-amino-4,6-dichloro-s-triazine over thin films of a cobalt catalyst patterned on a silica substrate by laser etching. The use of a patterned catalyst apparently encourages the formation of aligned nanotubes. The method offers control over length (up to about 50 mu m) and fairly uniform diameters (30-50 nm), as well as producing nanotubes in high yield, uncontaminated by polyhedral particles.