Street-level and rooftop (three-story building) concentrations of particulate matter < or = 2.5 microm in diameter (PM2.5) were measured in downtown Trujillo, Peru, in July and August 2003 to determine the PM2.5 concentration reduction on days with normal traffic conditions (32 days) versus a day when motor vehicles were temporarily banned from the downtown district (8:00 a.m. to 6:00 p.m., July 15) known as El Dia Sin Autos ("The Day Without Cars"). The mean 8:00 a.m. to 6:00 p.m. street-level PM2.5 concentration during the motor vehicle ban (21.4 microg/m3; one day) was 49% lower than when vehicles were not impeded (42.2 +/- 7.8 microg/m3--mean +/- 1 standard deviation; 20 days). The rooftop monitoring station indicated a 20% decrease in PM2.5 concentrations (24.8 +/- 2.6 microg/m3 vs. 19.9 +/- 6.0 microg/m3) when motor vehicles were not present within historic downtown Trujillo. Temperature, relative humidity, and wind speed during the motor vehicle ban and during normal traffic were not significantly different (p > .05).