Experiments were performed to measure the average and time-resolved particle number emissions and number-weighted particle size distributions from a gasoline direct injection (GDI) engine. Measurements were made on a late model vehicle equipped with a direct injection spark ignition engine. The vehicle was placed on a chassis dynamometer, which was used to load the engine to road load at five different vehicle speeds ranging from 13 - 90 km/hr. Particle number emissions were measured using a TSI 3020 condensation nucleus counter, and size distributions were measured using a TSI 3934 scanning mobility particle sizer. Average polydisperse number concentration was found to increase from 1.1 × 108 particles/cm3 at 13 km/hr to 2.8 × 10 8 particles/cm3 at 70 km/hr. Under a closed-loop, stoichiometric homogeneous charge operating mode at 90 km/hr, number emissions fell to 9.3 × 107 particles/cm3 (at all other operating conditions, the engine was in a lean stratified charge operating mode). Time-resolved number emissions were found to be steady at 13, 50, and 70 km/hr, unsteady at 90 km/hr, and exhibited periodic behavior at 32 km/hr. For the operating conditions ranging from 32 - 90 km/hr, the number weighted size distributions were well represented by broad (geometric standard deviation ∼ 1.8) monomodal lognormal distribution functions. The number-weighted geometric mean diameter ranged from 68 nm at 32 km/hr to 88 nm at 90 km/hr. For the 13 km/hr operating condition, the size distribution was bimodal. In addition to the broad accumulation mode observed at the other operating conditions, the 13 km/hr condition also contained a very strong nuclei mode with a geometric standard deviation of approximately 1.3 and a number-weighted geometric mean diameter of approximately 20 nm.