Many metropolitan school districts worldwide operate bus fleets with older diesel school buses. Emission reduction strategies such as the diesel retrofits promise substantial reductions in particulate mass emissions for older diesel engines. Recent studies suggest, however, that particle number concentration is a more important factor than particle mass in developing emissions standards and predicting adverse health effects. In-vehicle particle number concentration measurements on clean diesel, retrofitted diesel, and non-retrofitted diesel school buses were compared to estimate retrofit effectiveness in reducing in-vehicle passenger particle exposures. The retrofits were shown to provide a 15% to 26% decrease in in-vehicle particle number concentration levels between the retrofitted and non-retrofitted buses. Furthermore, the retrofitted diesel buses had average in-vehicle particle number concentrations near the average concentrations for the newer model, clean diesel buses. The average particle number concentrations for the non-retrofitted diesel buses were 1.5 to 2 times higher than the concentration measured from the retrofitted diesel buses. Moreover, a two-fold increase was observed for the morning commute samples over the afternoon commuter samples. Particle number concentrations were also significantly affected by engine age, bus idling behavior, and ambient particle concentrations along bus routes.