Particle mass concentrations and size distributions have been measured upstream and downstream of a ceramic-coated metal fiber filter. The filter has been used to process the exhaust of a 1.5 liter light-duty indirect injection diesel engine. At light filter loadings, the collection efficiency was typically found to range from 50 to 60%. As the trap was loaded, the efficiency slowly increasesd to a maximum value between 70-80% and then slowly declined. At the same time, the pressure drop across this filter continuously increased. The trap was periodicaly regenerated to prevent excessive pressure drop. When lightly loaded, the trap was found to reduce the particle concentration without having a significant impact on the size distribution. However, a heavily loaded trap significantly shifted the particle size distribution upward. Under some conditions, the trap actually generated coarse, greater than 1.0 μm diameter, particles. The trap thus acts as a particle agglomerator. This behavior is explained in terms of collection, surface agglomerataion, and subsequent reentrainment of the agglomerated material.