Single-walled carbon nanotubes (SWNTs) give every indication that they will make a major impart on science and technology. Their unusual physical and chemical properties, which depend on nanotube diameter and helicity, make them particularly attractive for future molecular nanotechnology applications. Production of a variety of nanotubes will be necessary to explore novel properties arising from a variety of derailed atomic structures. We have synthesized large-diameter SWNTs by adding catalyst promoters, using either Co or Fe as primiary catalyst. SWNTs produced by transition metal catalysts alone have a diameter distribution between 1 and 2 nm. Adding catalyst promoter alters the diameter distribution to a much wider range, with a variety of helicities, and dramatically increases the nanotube yields. The results indicate the need for a general growth mechanism that accounts for the effect of catalyst promoters.