Particle concentrations and size distributions were measured in the exhaust of a turbocharged, aftercooled, direct-injection, Diesel engine equipped with a ceramic filter (trap). Measurements were performed both upstream and downstream of the filter using a two-stage, variable residence time, micro-dilution system, a condensation particle counter and a scanning mobility particle sizer set up to count and size particles in the 7-320 nm diameter range. Engine operating conditions of the ISO 11 Mode test were used. The engine out (upstream of filter) size distribution has a bimodal, log normal structure, consisting of a nuclei mode with a geometric number mean diameter, DGN, in the 10-30 nm range and an accumulation mode with DGN in the 50-80 nm range. The modal structure of the size distribution is less distinct downstream of the filter. Nearly all the particle number emissions come from the nuclei mode, are nanoparticles (Dp < 50nm), and are volatile. The emission of nanoparticles downstream of the filter is influenced strongly by the residence time; changing the residence time from 40 ms to 6000 ms increases the number concentrations of nanoparticles by up to three orders of magnitude. Residence time also considerably influences the engine out number concentrations and the shape of the size distributions, especially in the nuclei mode range. In ISO mode 8, total number concentrations vary from 47 to 600 to 311x106 part./cm3 as residence time increased from 40 to 1000 to 6000 ms.The number weighted filter penetration is highly dependent on residence time. At a residence time of 40 ms, penetrations are less than 1% for nearly all operating modes, but at a residence time of 6000 ms, modes 1,2,3, and 6 gave fairly high overall penetrations of 60 %, 73%, 130 %, and 47 %, respectively. These high penetrations result from particle nucleation downstream of the filter, not inherent filtration performance.The filter self-regenerates when the engine is operated at the rated power condition (ISO mode 1). During regeneration, emissions of particles below 13 nm in diameter increase by more than three orders of magnitude for a few minutes.Particle nucleation and growth during dilution are poorly understood and strongly influence measured number emissions and size distributions. More work is needed to establish consistent, well-defined sampling and dilution procedures for measurements of particle number emissions and size, especially in the nanoparticle size range. These procedures should, to the extent possible, mimic atmospheric dilution conditions so that measurements are representative of real world exposures rather than arbitrarily defined laboratory conditions.