Particulates were collected from the exhaust of a single cylinder diesel engine at different speeds and fuel-air equivalence ratios. The filters were rated to capture 99.99% of all particulates >0.3 microns in diameter. Samples were taken at 1500 RPM and equivalence ratios varying from about 0.2 to 0.7, and at an equivalence ratio of about 0.3 and engine speeds varying from 1000 to 2500 RPM. A base point with an equivalence ratio of 0.3 with an engine speed of 1500 RPM was repeated several times to establish the expected test variation of the particulate data. The particulate properties investigated were the total mass of particulate produced per mass of fuel burned, the mass fraction of extractable organic material in the sample, and the mutagenic potency of the extract as measured by a bacterial bioassay.Variation in fuel-air ratio (engine load) affected the particulate and extractable organic production much more than variations in engine speed. The ratio of mass of particulates produced was three to Four times higher at both high and low equivalence ratios, than in the equivalence ratio range between 0.3 and 0.6. At low equivalence ratios, the particulates had a soluable fraction approximately five times higher than mid-range. The soluable fraction decreased rapidly until the equivalence ratio reached 0.4 remaining relatively constant above that point. The mutagenic potency of the extract was constant across both speed and equivalence ratio ranges within the accuracy of the bioassay.