Time resolved primary and agglomerate particle size distribution measurements have been made on samples obtained from within the cylinder and from the exhaust of a single-cylinder modification of a 2.8 liter displacement, four-cylinder, naturally-aspirated, high swirl, direct-injection diesel engine. The total cylinder sampling method has been used to sample, quench, and dilute the entire contents of the cylinder in about 1 ms. Experiments have been performed at an equivalence ratio of 0.7 and a speed of 1000 RPM. An electrostatic aerosol sampler and a transmission electron microscope have been used to determine primary and agglomerate particle size distributions for both in-cylinder and exhaust samples. An electrical aerosol analyzer and a diffusion battery followed by a condensation nucleus counter were used to further characterize the agglomerate size distributions of exhaust samples. Geometric number mean diameters of primary particles and agglomerates have been determined for exhaust samples and as function of crankangle for in-cylinder samples. The earliest particles to appear are agglomerates composed of a few primary particles. These grow into larger agglomerates composed of hundreds and thousands of primary particles by coagulation. At the same time the diameter of the primary particles decreases. Thus coagulation and oxidation appear to be taking place simultaneously.