This study was conducted to assess the effects of a low sulfur (<0.05 wt.%) fuel and an uncatalyzed ceramic particle trap on heavy-duty diesel emissions during both steady-state operation and during periods of electrically assisted trap regeneration. A Cummins LTA10-300 engine was operated at two steady-state modes with and without the trap. The exhaust trap system included a Corning EX-54 trap with an electrically assisted regeneration system. Both regulated emissions (oxides of nitrogen - NOx, total hydrocarbons - HC, and total particulate matter - TPM) and some unregulated emissions (polynuclear aromatic hydrocarbons - PAH soluble organic fraction - SOF, sulfates, vapor phase organics, and mutagenic activity) were measured during baseline, trap, and regeneration conditions. Emissions were collected with low sulfur (0.01 wt.%) fuel and compared to emissions with a conventional sulfur (0.32 wt.%) fuel. These fuels also varied in other fuel properties.NOx emissions were found to be dependent on mode and fuel. TPM and SOF concentrations did not vary between the fuels and were dependent primarily on mode and trap use, while sulfate levels were typically below detection limits with the low sulfur fuel. HC and semivolatile organics levels were found to be dependent on mode, trap use, and fuel. Reductions typically occurred in TPM-associated PAH concentrations with trap use but not necessarily in vapor phase PAH. Higher TPM and vapor phase-associated PAH levels were typically found with the low sulfur fuel. Particle and vapor phase-associated mutagenic activity was typically higher at the lighter load mode and with the low sulfur fuel and decreased with trap use. Emissions during the trap regeneration process contributed less than one percent to the overall trap emissions. Some of the observed differences in emissions with the two fuels are most likely related to parameters other than fuel sulfur content, such as fuel chemical composition.