Recent work has indicated the presence of carbon nanotubes (CNTs) in laboratory diesel and gasoline exhaust, in ambient air, and in lung samples of children exposed to traffic exhaust. While it is already known that certain processes will produce some carbonaceous particles of fullerene-like crystallinity, the conditions responsible for their formation remain unknown. On the basis of a standard process for the gas-phase synthesis of CNTs, we hypothesized that the presence of a metal catalyst precursor and high levels of fuel sulfur would impact CNT formation in a diesel engine. A diesel engine was doped with varying concentrations of fuel-borne sulfur and ferrocene to produce conditioned iron (Fe) particles that acted as seed catalysts. Results showed that in the presence of Fe nuclei resulting from 36 ppm ferrocene doping, 4500 ppm of fuel sulfur produced CNT-like structures in 31% of images analyzed by transmission electron microscopy. The precursor concentrations required for high rates of CNT growth are comparable to those found in transportation fuels used in many regions of the world. These findings substantiate studies that indicate a global presence of CNT-like particles in ambient air. Formation of these structures is less likely with low-sulfur fuels, and the structures are effectively removed by particulate filters.