We have synthesized single-layer carbon nanotubes by co-vaporizing cobalt with carbon in an are fullerene generator and have identified conditions that lead to high yields. The diameter distribution of the tubes and their morphologies are studied using transmission electron microscopy. For nanotubes produced using cobalt and carbon, the tube diameters range from 1 to 2 nm with distribution peaks at 1.3 and 1.5 nm. When sulfur is added to the carbon and cobalt, production of single-layer nanotubes is enhanced and the tubes have a wider range of diameters (from 1 to 6 nm). The diameter distribution for these nanotubes shows prominent peaks al 1.3 and 1.5 nm and additional maxima at 2.7 and 3.6 nm. Cobalt-containing crystallites, some encapsulated in graphitic polyhedra, are produced with the nanotubes and are found in the soot away from the cathode.