From 2002 to 2007 fuel sulfur content in the Minneapolis/St. Paul area decreased from about 325 ppm S to <15 ppm S as a result of EPA regulations. We hypothesized that the reduction in fuel sulfur would result in a reduction of fuel specific heavy duty (HD) particle number emissions for the onroad diesel fleet. Fuel specific emissions were estimated by collecting on-road aerosol data, and exploiting the difference in the relative volumes of HD and light duty (LD) traffic on the roadway. Fuel-specific particle number emissions forHDvehicles were measured to be 9.1 ± 6.6 × 1015 and 3.2 ± 2.8 × 1015 particles/kg of fuel burned, in 2006 and 2007, respectively, a reduction of 65%. In an earlier study in 2002 particle number emissions for the in-useHDfleet were 4.2 ±0.6×10 15 particles/ km compared to the current measurements of 2.8 ± 2.1 × 1015 and 9.9 ± 8.7 × 1014 particles/km in 2006 and 2007, respectively. TheHDparticle mass emission standard remained unchanged from 1994 through 2006 and few 2007 HD vehicles were on the road at the time of this study so the decreases in number observed emissions are more likely due to reductions in the sulfur content of the fuel than to changes in engine and aftertreatment design.