Diesel exhaust particle size distributions were measured using an Electrical Aerosol Analyzer (EAA) with both conventional (0.31 wt. pet sulfur) and low sulfur fuel (0.01 wt pet sulfur) with and without a ceramic diesel particle filter (DPF). The engine used for this study was a 1988 heavy-duty diesel engine (Cummins LTA10-300) operated at EPA steady-state modes 9 and 11.The particle size distribution results indicated the typical bi-modal distribution; however, there were clear differences in the number of particles in each mode for all conditions. For the baseline conditions with no DPF, there was more than one order of magnitude greater number of particles in the nuclei mode for the conventional fuel as compared to the low sulfur fuel, while the accumulation modes for each fuel were nearly identical. The effect was even more pronounced with the DPF data, which indicated the number of nuclei-mode particles with conventional fuel and DPF was more than two orders of magnitude greater than with low sulfur fuel and DPF. These results indicate the nuclei-mode particles for all conditions may in fact consist of a large percentage of sulfate particles and/or that the sulfate particles may act as nuclei which are necessary for a hydrocarbon nucleation process to occur.