Particle size distributions have been measured in the exhaust of a single cylinder Onan diesel engine using an electrical aerosol analyzer. These measurements give volume mean diameter for the exhaust particles of about 0.1 μm. Other investigators have shown that the particles found in diesel exhaust consist of agglomerates of very small primary particles (about 0.025 μm diameter) and may contain condensed hydrocarbons. A mathematical model has been constructed to determine the particle size distributions which will result from the growth of the primary particles by coagulation. The coagulation equation was solved numerically for an expanding stratified system. The model indicates that the inhomogeneity characteristic of stratified combustion can explain the rapid growth of the primary particles into the larger particles observed in diesel exhaust.